An odd phenomenon in some video games with guns in that any ammunition you pick up works with either any gun whatsoever or an entire class of guns. In the real world, cartridges have a variety of different calibers, lengths and propellants based on a number of factors, including desired stopping power, target penetration, and the amount of recoil and gas pressure needed for a semiauto or automatic weapon to cycle itself (or, the other way around, prevent it from exploding in your face).
These cartridges are then loaded into clips and magazines designed for specific weapons, so that even if the cartridges are identical between two or more guns, you'll have to unload and reload one painstaking round at a time before you can take advantage of it; nevermind the question of where you're getting the empty magazines from. Sitting down and filling up mags is "not always an option" in a pitched firefight, and largely qualifies as an Acceptable Break from Reality.
In fictional settings, such as science fiction, this is sometimes purposely justified by declaring there has been a worldwide standardization causing an actual adoption of universal ammunition or some sort of literally universal ammunition that's capable of reconfiguring itself to completely differing weapons.
Can also be a Justified Trope in modern shooters, as many military and civilian weapons very intentionally use common "standard" ammunition sizes (like the NATO 5.56 and 7.62 rounds for assault rifles) or certain calibers of ammunition tend to be overwhelmingly popular for certain guns (the .40 S&W and 9mm Luger loads for pistols), and the ranges at which video game firefights usually occur are in real terms absurdly short ("sniping" at 50 meters or less is a common "long range" option), making the difference in propellant loads rather moot due to basically everything flying completely flat at 5 or 10 meters (a typical range for everything that's not "sniping").
Note that most Energy Weapons batteries/power sources, in contrast to the panoply of batteries in Real Life, are portrayed as universal. Just pop in a random battery and they're good to go.
Before adding examples please keep in mind that a lot of weapons from video games are based on real-life weapons. When two weapons which use compatible magazines in real life use the same ammunition in a game it is not an example— though if those two weapons use different ammunition in the game, it is an averted example.
See also Bottomless Magazines, One Bullet Clips.
Inexplicable video game examples:
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Golden Eye 1997 has two main types of ammo, pistol and rifle. No matter what pistol or submachine gun you use, it'll take the same ammo. Likewise, no matter what rifle you pick up, it'll take whatever rifle ammo you have. However, there are unique types of ammunition specific to certain weapons such as rocket ammunition for the rocket launcher or shotgun shells for the shotgun.
Medal of Honor: Allied Assault is in the same boat, with wildly different guns from four nations all accepting the same carts and mags if they're the same "type" (pistol, rifle, etc.).
The original PSX game averted this for pistols and sub-machine guns (for instance, salvaged 9mm guns would not give you ammo for your .45 Colt or Thompson), but played it straight with rifles.
Perfect Dark and its prequel added a category for SMG ammo, but otherwise retained the same ammunition setup as in GoldenEye. Even funnier is that there are some alien weapons in the first game which draw from these pools (and some of them even have reloading animations of putting an extra glob of goob into the gun!) - good thing the Skedar chambered their handguns for 9mm, right?
TimeSplitters also used a similar system (since the game was developed by the same people as GoldenEye).
The game version of The World Is Not Enough has a similar system - ammo types are, for the most part, identified by caliber, and are only shared by weapons that share them in real life (ignoring the One Bullet Clips issues that result). However, this still results in the AKS-47 inexplicably loading the same ammo as the 7.62mm NATO sniper rifles.
In Team Fortress 2, picking up any gun dropped by a dead player gives your current weapon more ammo. Even if it's something like the Pyro's flamethrower or the Soldier's rocket launcher. Even scraps of a destroyed engineer building converts into ammo.
It will also refill the spy's cloak meter and provide the Engineer with extra Metal for buildings. The latter makes sense while the former... really doesn't.
And that wooden bats and broken bottles not only provide metal, but just as much metal as the Heavy's enormous minigun.
An exception is made for the Sandvich and its variants, who simply heal the player that collects them. Not the case with the Scout's energy drinks or the Jarate.
Lampshaded hilariously in Poker Night at the Inventory. According to the Heavy, his minigun uses ammo with classified diameters so that his enemies cannot use it... but it can use theirs?
Averted to a slightly unrealistic degree at times in the earlier Call of Duty series. Even the normal and sniper versions of a weapon did not have interchangeable ammunition; while in some cases like most bolt-action rifles this was at least semi-accurate since attaching a scope made it impossible to reload them using stripper clips, it was even the case when the weapons should have used exactly the same ammunition loaded exactly the same way, like the Sten being nearly-impossible to replenish despite taking the same magazines as the MP40s every other German you kill with it drops, or a scoped Mosin-Nagant reloading with clips anyway but not being able to take more from unscoped ones.
Call of Duty 4 changed this around and would allow you to take ammo from any dropped weapon if it fired the same kind of bullet as what you already had, and if it isn't already the exact same gun you're using, the gun itself would stay on the ground in case you wanted to swap for it. You can also increase the amount of maximum ammo for a gun by carrying another one that fires the same kind of bullet (such as the 5.56x45mm M16, M4, and/or G36C, which falls into this when you're using two guns that are clearly not using the same magazines). Singleplayer mode in later games is a bit stricter, where you can only take ammo from another gun if it's the same base model (i.e. an M4 with any attachment[s] would give ammo to any other M4, but not to an M16; this is usually not very limiting, however, as the player's starting gun usually has more attachments and reserve ammo than is normally possible or even necessary to complete the level, and anything you could replace it with would get more ammo from every enemy or ally that dies during the level). In multiplayer however, extra ammo only comes from exact copies of the gun you have, meaning that if you were carrying an AK-47 with Red Dot Sight and Red Tiger camouflage, you can only pick up ammo from another AK-47 with Red Dot Sight and Red Tiger camouflage. Basically, your only realistic hope for more ammo is finding weapons you dropped yourself or taking guns off of people you kill.
Played straight with the "Scavenger" perk - every casualty will drop a backpack with one full magazine for whatever guns you're carrying, another piece of your primary equipment, and one special grenade. In Modern Warfare 2, this backpack also contains a 40mm grenade or four shotgun shells if your gun has the grenade launcher/shotgun attachment. Yeesh, good thing the enemy didn't know they were carrying all this stuff...
Turning Point: Fall Of Liberty suffered from a glaring lack of ammo commonality. No two of the game's many 9mm weapons could share ammo. In fact, if you had the scoped and unscoped versions of the same gun, they still do not share ammo. Did we mention the game was an Obvious Beta?
In Enemy Territory Quake Wars, the Strogg use the exact same pickup for all their ammo and their health restoration. This would normally fall under Justified Examples... except that the pickup in question was established in Quake IV to be a specially treated slurry of liquefied human remains, and could not conceivably function as ammunition for anything except a particularly disgusting squirt-gun.
Left 4 Dead justifies this: all guns reload their ammunition or get upgraded ammo from the same pile/box, but they are clearly shown to contain a variety of ammo. There are also a few weapons that cannot be reloaded from the ammo piles but can get explosive or incendiary ammo, specifically the grenade launcher and M60.
In Far Cry, the MP5 and P90 use the same ammo, with boxed "SMG" ammo found in the game having "9x19" written on it. Right ammo for the MP5, but absolutely wrong for the 5.7x28mm P90.
In Far Cry 2 and Far Cry 3, all weapons of the same type use the same ammunition, even when taken from picked up weapons. This means, for example, the pistols are simultaneously chambered in .45 ACP, 9mm Makarov, .50 Action Express, and (in 3) .44 Magnum. They apparently tried to justify this for the primary weapon slot in 2, where the assault/battle rifles all use 7.62mm bullets in real life, even though one of them (the AK-47) doesn't use the same 7.62 bullets as the others; conversely, this also means that the player can be carrying two 40mm grenade launchers in the second game and not share ammo between them for no other reason than one of them is being used in place of a pistol and the other in place of a rifle or shotgun.
Half-Life's pistol and submachine gun use the same ammunition. This isn't a problem with regards to the Glock and MP5, using 9mm pistol ammo, although Fridge Logic arises in how Gordon transfers the ammo between guns (he likely does so the same way he transfers bullets between half-used magazines - that is, his HEV suit does it for him). However, the Hi-Def pack - an overall graphical upgrade - changes the Glock into a Beretta and the MP5 into an M4 assault rifle. That creates a new problem - the M4 uses 5.56mm rifle ammo, not pistol ammo. Another problem comes when combining the Opposing Force expansion with the Hi-Def pack, which adds an M249 SAW that should share ammo with the M4, but doesn't. This was fixed in the PlayStation 2 port, which used the hi-def models and added a separate ammo type for the M4.
Totally averted in Half-Life 2, as every weapon uses unique ammunition that even has a unique world model (so the submachine gun ammo appears as an ammo box correctly labeled as holding 4.6x30mm ammo). One Bullet Clips still apply.
Averted in STALKER, where there there are some guns that use unique caliber suitable only for them, not to mention each gun has two or three different types of ammo for various situations.
Although a criticism with this is the 9x19mm (9mm Parabellum) looks a little too similar to the 9x18mm (9mm Makarov), both in the in-game name and icon. Some players not used to this may wonder why they can't use one or the other when they're both 9mm. The game does occasionally make up for this with alternate versions of guns chambered for a different, more plentiful ammo type (such as a unique MP5 firing 9mm Makarov) and in the games with upgrades, most guns using either round can be converted to use the other.
Exhumed (a.k.a. Powerslave) has a variation in the Sega Saturn and PlayStation versions in which the ammo pickups are all generic blue orbs that refill whatever weapon you are holding at the time, but the weapons themselves all have different ammo pools beyond that.
In Guns Of Icarus Online, players can equip special ammunition to apply various advantages and disadvantages to their weapon. Special ammo works in any weapon, regardless of whether the weapon is firing bullets, missiles, or burning gasoline.
PlanetSide 2, unlike its predecessor, uses universal ammunition for its weapons. An engineer dropping an ammo pack will refill anyone's ammo; be it the slugs for a Gauss Rifle, caseless ammo for the Mini Chaingun, the plasma batteries for a Lasher, or the high-explosive rockets for a Decimator.
In Duke Nukem: Manhattan Project, you have 3 different Ammo Slots: Ammo, Pipe Bomb, and Energy. Ammo is used for your Desert Eagle, Shotgun, and Assault Rifle, though the minor pickups only show 3 Desert Eagle rounds (the Major pickups show one of each round). Pipe Bombs are used for Pipe Bombs and the RPG. Energy is used for the De-Mutator (a glob throwing weapon that de-mutates any mutants you run across) and Pulse Rifle.
Real Time Strategy
In Star Ruler, all ships draw from an abstracted ammo supply with no attempt to distinguish types. Given that you can have any number of ships in varying sizes from fighters to planet-sized (or bigger!) super-battleships, trying to manage individual ammo types would be a real pain in the arse anyway.
Averted, since you custom-make all of his weapons yourself, with the exception of his starting weapon, which the first upgrade is created from by using it as a base. In this case, you're modding the rifle to the ammo, most of which you can make yourself as well. Double averted?
All weapons in Fallout that use the same caliber take the same ammunition. The issue of different cartridges in the same caliber is ignored, magazines/clips/belt links don't need to be kept track of, and characters seem to load mags in the heat of combat.
Possibly justified by the fact in a world with definitely limited ammunition guns would be modified to accept the most common types.
Fallout 2 has a car that can be recharged by both Small Energy Cells and Fusion Batteries - themselves used as clips for a wide assortment of energy weapons.
Due to a bug in Fallout 2, the P90 expy gun was originally loaded with 9mm ammo, which was extremely rare and only (designed) to be used in this one gun (it wasn't even the only specialized 9mm ammo, either - elsewhere in the game is a Mauser that takes 9mm (ball) ammo). However, once emptied, it then took extremely common 10mm ammo.
In Fallout 3, Lincoln's Repeater (an authentic Henry Rifle) uses the same ammo as the .44 revolver. A .44 Magnum round might fit inside a Henry rifle, but the rifle's pin would hit the wrong part of the cartridge, missing the primer entirely and thus failing to fire. Even if it could fire, the higher pressure would likely destroy the rifle in short order.
Fallout: New Vegas has several weapons which share ammo pools, some more sensibly than others. For instance, it's not too far out of the question that the 9mm SMG uses the same round as the 9mm pistol, but there's a few questions as to why a superheavy machine gun loads handgun rounds and two of the miniguns load revolver rounds. The most questionable universality in ammo might be that of the flame weapons: weapons that fire flamer fuel can somehow use this ammunition type to produce a flamethrower's flames, an incinerator's arcing fireballs, or most oddly, a flare gun's magnesium flare.
A high enough Science skill will allow you to use Work benches to convert the three main types of energy weapon ammunition between each other, effectively invoking this trope yourself. You can also transfer the energy in fission battery to small energy cells (then covert them to whatever type you need)—which mysteriously creates five charged cell for every drained one the recipe requires.
All firearms in Arcanum: Of Steamworks & Magick Obscura, barring a few exotic examples, take the same bullets. Larger guns just shoot more than one at a time. Of course, the Bullets schematic only requires the ingredients for black powder, so it's unclear just what exactly you're shooting.
On a larger scale in EVE Online, one unit of a certain "size" of projectile ammunition can be chambered in up to five calibers. For example, one unit of "Large projectile ammunition", meant to be fired from battleship guns, can be used interchangeably in Dual 425mm, 650mm, 800mm, 1200mm, and 1400mm guns. Also, a unit of ammunition unloaded from a 800mm autocannon takes up the volume as ammunition unloaded from a 1400mm howitzer.
Averted in 7.62mm High Caliber, with each gun taking the proper caliber and many calibers coming in multiple brands and types that can be mixed and matched in the magazines, and each gun takes its own magazine. One error is that the Mauser pistol and carbine take Tokarev ammo (which will load in a Mauser but are too powerful to safely use, and require 7.63mm Mauser ammunition instead).
Played absurdly straight in Persona, where your first ammo pickup is "9mm bullet" (followed by "12ga bullets" before moving on to "Tranquilizers", "Mage Killers" and so on) which you can equip with any gun type, be it an SMG, a shotgun, or an assault rifle.
Averted in Operation Flashpoint and its Spiritual SuccessorARMA. Unless the type of ammunition is used in a whole closely related family/series of firearms, you'll have to find appropriate ammo for each gun. You won't have much luck firing an AK-74 with a magazine of M16 bullets. As noted, however, related firearms can take the same ammo (most assault rifles of Western origin in the series use the same magazines as the M16, for example).
Resident Evil 4: Ammo, for the most part, is totally interchangeable within any weapon of a certain classification. This leads to the 5.7x28mm Five-seven firing 9x19mm Parabellum, and the .30-06 Springfield and 5.56x45mm SL8 sharing ammo (though the inventory screen flat-out states the Springfield is firing .223, so it could have been converted).
Furthermore, the magazines are the same size for each gun, but the ammo cap for each gun can be upgraded. Is Leon putting two mags in on top of each other?
Though justified in the main game of Resident Evil 2 (both Leon and Claire's starting handguns do, in fact, use the same type of ammunition in real life), it is invoked poorly with Claire's bonus weapon, the Colt .45 revolver. Granted, it's to fit the whole "cowgirl" theme along with her outfit, but as its name suggests, the revolver uses .45 LC caliber rounds, not 9mm rounds.
It's invoked even more poorly in Resident Evil Gun Survivor, where all four of your available handguns use 9mm rounds, which includes the Nambu pistol, a gun that was chambered exclusively in 8mm. This is very weakly Hand Waved by the gun being described as "Custom," even though a Nambu would likely blow apart if one ever did go to the trouble of rechambering it for 9mm.
The first Parasite Eve game had this. The only unique ammo was the rocket launcher. Otherwise, you just had generic bullets. 9mm Parabellum = .45 ACP = 12 gauge = 7.62x51mm = 5.56x45mm = 40mm. For those who don't know ammo, pistol ammo = bigger pistol ammo = shotgun shell = rifle ammo = smaller rifle ammo = grenade.
Averted in Parasite Eve 2, each ammo type is Color-Coded for Your Convenience - this even gets lampshaded by one of the NPCs, who notes that another character jammed a pistol by trying to force the ammo from a different pistol caliber into it.
The third game in the Dead Space series exchanges the previous games' "each gun has its own ammo type" system for this trope. Partially it was explained as a simplification for the new weapon crafting system, but the fandom reacted with They Changed It, Now It Sucks (coupled with Lying Creator, as the initial Word of God was that this was only there for the demos).
Unturned plays with this. All guns use specific magazines that have to be pre-loaded — for example, the Maplestrike uses NATO mags and the Timberwolf uses Lapua mags. However, the magazines are reloaded with boxes of a broader type of generic ammunition - both the Maplestrike and Timberwolf use military bullets. The number of rounds provided by a box varies by the magazine.
In darkSector this is played pretty much straight with everything but the starting pistol; the Klin PP9 (9x18mm), AKS-74U (5.45x39mm) and G36C (5.56x45mm) all share an ammo pool, as do the Webley Mark IV revolver (really a .38, but supposed to be a Nagant, firing 7.62x38mm rimmed) and the SOCOM 16 rifle (firing 7.62x51mm NATO).
Zig-Zagged by Tomb Raider. Each of Lara's weapons (bow, handgun, shotgun and rifle) use a different type of ammunition. However all upgrades use their same respective ammunition. This is justified with the bow and shotgun, but is played ridiculously straight with the pistol and rifle: Lara replaces her Beretta 92 (9x19mm Parabellum) with an M1911 (.45 ACP) and then somehow upgrades that M1911 into a Desert Eagle (.50 Action Express), while the rifle starts as a Type 100 (8x22mm Nambu) and is eventually turned into an AK-47 (7.62x39mm Soviet) and then some strange combination of it and an Ultimax 100 Mark 3 (5.56x45mm NATO); in both cases the very same ammo pickups will give you bullets for these guns regardless of what stage of upgrading you have them at. She also uses the same grenades for the rifle's grenade launcher to create "grenade arrows" (though that one is arguably justified). Also, all previous modifications (padded stocks, port-vented slides, etc.) automatically carry over when Lara upgrades her weapons - even in the above case of replacing her Beretta with the 1911 rather than upgrading it into one.
In Red Dead Redemption, each of its 21 firearms are in one of 6 categories: Revolvers, Pistols, Rifles, Repeaters, Sniper Rifles, and Shotguns. Each category has an associated ammunition type (i.e. shotgun ammo for shotguns) which is universal across all weapons in the category.
Inconsistently applied in Sniper Elite V2, where you can search the corpses of enemies to get ammo, but each weapon has its own ammo pool and SMG ammo is almost never found in this manner. You can typically find American .30-06 or Russian 7.62x54mmR rifle rounds on a dead German who was armed with an MP 40, but you can't find 9x19mm rounds for that very same MP 40 of his if you decide to take it. Or, worse, you can, but they'll be only usable in the Welrod or Luger pistol instead. This gets even worse with weapons added through DLC, where that same dead German may now have British .303 or even Japanese 7.7x58mm rounds on him for some reason.
In Jagged Alliance 2 all 7.62mm calibre weapons of WarPac/ChiCom origin use the same ammunition - 7.62mm, which would be logical, except there are three kinds of 7.62mm Warsaw Pact ammunition in real life: 7.62x54mmR, as used by the Dragunov and most of their MGs, 7.62x39mm as used by the AK-47/AKM and their relatives, and 7.62x25 used by the PPSh/Type 65 and other pistol type weapons. They are definitely NOT interchangeable; the second numbers denote case length, which means the first one would be more than twice the size of the chamber for the last. This was eventually fixed in the v1.13 modification.
While normally being pretty faithful to Real LifeWorld War II weaponry, Silent Storm does have a few odd-balls. For example, some submachine guns can be reloaded by equivalent bullets used by machine guns (submachine gun mags take up 2 vertical Grid Inventory squares, while machine gun clips take up 4). One has to wonder how a large circular magazine fits into the normal-sized mag housing. The same is also true for the Energy Weapons that become available late in the game. The handheld versions are bazooka-sized, but there are also energy-armed Panzerkleins. All the mags for these are large (4 squares) but carry only a charge for a single shot. However, an extremely-rare Random Encounter can result in you getting your hands on an energy assault rifle, which uses rifle-sized magazines (1 square) with 50 shots in each. Once you give the rifle to the quartermaster as the base, you can get unlimited 50-shot mags that also work with any energy weapon. So, instead of lugging around huge canisters that only offer you a single shot, you can carry 4 mags for 200 shots in the same space.
Borderlands has over 17 million guns, with Universal Ammo for each type of gun. The same bullets do different things, from explode to multiply, based on which gun they are loaded into. The second game suggests that a technology known as 'digistructing' might be the cause of this; specifically, Tediore guns are discarded explosively with a fresh copy built anew in the user's hands, complete with a fresh load of ammunition for whatever type of gun it is. It isn't too hard to believe that this might be the case for all weapons simply creating fresh ammo in magazines from a generic supply on your person.
The Vanu Sovereignty in PlanetSide chamber the same type of ammunition (batteries) for their main weapons, as they all need similar levels of power. Their anti-tank laser cannon however, requires a much larger battery pack. Almost all handheld projectile rifles and pistols are chambered in 9mm ammo, bar sniper rifles, allowing a Terran Republic soldier behind enemy lines to reload his Mini Chaingun with ammo looted from New Conglomerate Gauss Rifles, for example.
Deus Ex: Invisible War takes this trope to the extreme. Anything that uses ammunition uses the same ammunition as everything else. The in-game explanation is that ammunition is reduced to a slurry of nanomachines that form into the proper ammunition for the weapon. This includes battery packs, flame thrower fuel, regular bullets, and even rockets. The creative reason behind this is that the inventory system got simplified, and carrying 20 types of ammo didn't sound appealing when you only had 7 spaces. This has the major drawback of burning your ammo with one gun means you're screwed.
Human Revolution averts this even further, as ammunition takes up inventory space and almost every weapon requires specific ammunition; only very similar weapons like the regular and double-barrel shotgun or regular and silenced sniper rifle use the same ammo. One Bullet Clips are still present.
The Blake Stone games have all the weapons using the same ammo. Justified in that all the weapons are energy-based and the ammo is energy packs. Just don't go too crazy with the more ammo-hungry guns that you have to resort to using your emergency self-charging pistol.
The Doom trilogy: the first two games had four types of ammunition for six guns (seven if you include the super shotgun). The Plasma Gun and BFG share ammo, which with them both being energy weapons powered by battery packs is justified. The pistol and chaingun also share ammunition, which is less justified. Doom 3 averts the trope entirely; every gun now uses separate ammo (except for, once again, the new super shotgun in Resurrection of Evil, which shares ammo with the base shotgun).
Wolfenstein 3D uses this trope, but to its credit, there's only three guns.
And the Luger pistol and MP40 SMG both use 9mm cartridges (though not from the same magazines.) The minigun? Well it is fictional.
Later ports of the game add a rocket launcher and flamethrower weapon, each of which use separate ammo types from each other and the regular guns.
Back to Half-Life: the gluon gun and tau cannon share the same (nuclear) battery packs. According to the Half-Life wiki, these weapons are powered by a revolutionary miniature nuclear reactor fueled with depleted uranium - the content of said battery packs.
Partially subverted in Dark Forces; despite all the weapons being energy-based, they use two different kinds of battery packs/cells. The main exceptions are the Dark Trooper assault cannon, which uses two different types of ammo exclusive to it, and the explosives, which are individual units.
The Battlezone games' Hover Tanks feed their machine guns, cannons, rockets, mines, extras from mines to the Phantom VIR and even energy weapons from the same "nano-ammo" supply.
Return to Castle Wolfenstein uses this for some of the guns; first for the Luger, MP40 and Sten all using the 9mm Parabellum rounds (the British designed the Sten specifically to use MP40 magazines for work behind enemy lines) along with the M1911 Colt pistol and Thompson both using .45 rounds, and the Mauser and FG 42 both taking 7.92mm rounds. The other guns, however, each have their own ammunition.
Serious Sam's shotguns always share ammo. The Minigun also shares ammo with the Tommy Gun in 1 and the Uzis in 2 - NETRISCA's info screen for them specifically mention the latter two are re-chambered for 5.56mm rounds, while the former is a man-portable version of a 5.56mm Minigun. Inexplicably, Sam 3 gives the minigun its own ammo supply, even though its Assault Rifle is still firing the same ammonote then again, the minigun this time is more blatantly based on the real-world M134, so the minigun ammo this time could be 7.62mm.
Metroid Prime Hunters have actual nanotech-based Universal Ammo that fits all of your special weapons. Only the missile launcher uses different ammo. Since all the guns are at least partially energy weapons, it's a bit more believable, though, as the ammo itself probably doesn't need to shape itself to extreme tolerances - even for the explosive Battlehammer, "close enough" will do, allowing a quick configuration to be at least somewhat plausible.
Mass Effect also takes this to the extreme. Ammunition in the game is nothing more than a metal block that gets shaved into the proper form. However, unlike above where it's crippling, this game has as-good-as-unlimited ammo, with rate of fire being limited by the weapon's heat sink. The block in a standard assault rifle is about 5,000 shots, which is more than enough to fight a war, and with field tools a new block can be swapped into place in a matter of minutes. In Wrex's back story, he once fought a duel that lasted three days. He ran out of ammo and had to take a gun from a merc he'd killed in the process.
Mass Effect 2 kept the unlimited ammunition, but added "thermal clips" that absorb heat from the weapon and eventually need to be discarded. Essentially, they act just like ammunition. The clips are universal - pick one up and it adds more total shots to all of your weapons. However, the game does feature a more straight example of the trope with the Heavy Weapons, which do use traditional ammo... the same ammo, whether the weapon is a rocket launcher, an experimental BFG, or an alien energy weapon built from technology beyond what anyone else has. That this "ammo" is actually a high-density power cell doesn't explain either its universal compatibility or the weapons' ability to form complicated projectiles from that raw energy - though it may be that, like standard mass effect weapons, at least the second point is rendered moot by the weapon typically having more than enough ammunition to go through a full load of energy dozens of times, and being serviced on return to the Normandy. Also, Shepard can run out of thermal clips for one weapon, yet still have plenty of shots left in the others, which is inconsistent with said ammunition being universal.
In Fallout 3, the .32 Caliber pistol and Hunting Rifle both use .32 caliber ammo, however the rifle is much more powerful than the pistol. The pistol is probably the weakest weapon in the game. Justified in that the rifle has a much longer barrel and a completely sealed chamber, and therefore can propel the .32 slug better then the tiny, unsealed revolver.
Fallout: New Vegas also allows weapons to share ammo, with differences in damage dealt per shot by them often depending more on what type of ammo is loaded into them rather than the weapon itself; overpressure ammunition (or overcharged/max charge energy cells and microfusion cells) deal greater damage and have better armor penetration. It's not even usually a clear choice, either, as Breakable Weapons is in effect and the most common ammo that deals more damage tends to be surplus bullets that degrade your weapons faster. Likewise, there are low power versions of cartridges (like .223 for 5.56mm weapons or .38 and .44 Special for .357 and .44 Magnum, respectively) that deal less damage but cause less degradation. There are even special hand loaded, match grade rounds and bullets like wadcutters or jacketed soft points that deal extra damage and have better armor penetration without degrading the weapon, and rounds that deal better damage (hollow points) or have better penetration (AP) while doing less of the other type of damage. Simply put, there's such a huge variety of ammunition that the game had to include reloading benches to let players reload their ammo with powder, lead, and primers (either purchased from vendors or taken from broken down ammo that you don't need to use) or build their preferred loads. A major part of the modding scene for New Vegas is adding more ammunition types and expanding the reloading aspect to ensure that everyone can make whatever they want.
New Vegas also inverts this with shotguns for the sake of Sorting Algorithm of Weapon Effectiveness: most common, early-game shotguns use 20-gauge shells instead of 12-gauge, even though only one of the three shotguns' real life bases was available in 20-gauge.
Most (though not all) small energy cell, microfusion cell, and electron charge pack-using weapons justify the universality by not strictly using the cells as ammunition — being energy weapons, the cells only provide the energy to power the attack, which makes it a great deal easier to universalize things and simplify logistics to just one of three types of ammunition. Much like ammo above, the player is also able to recycle used cells, convert one type to another, and buy or produce low-charged "bulk" cells (which reduce damage but also weapon degradation) or overcharged ones (which increase damage and degradation instead).
This was earlier implemented to much less outrage upon reception in Oni. Oni has two kinds of ammunition: Kinetic, which supposedly consisted of explosives/propellant and slugs, usable for everything from SMGs to rocket launchers, which configured them appropriately on loading; and energy, which was for things like typical plasma blasters and tasers. The bigger the gun, the fewer shots any given ammo will provide before being used up.
Resident Evil 4 again may fall into this. Among the weapons that share ammo are the Broken Butterfly revolver and the semi-auto Killer7, both of which take .45-caliber rounds. So long as they were rimmed cartridges, this would be entirely possible; revolvers will generally shoot anything you can fit into their cylinders.
The TMP, meanwhile, has its own unique ammo that despite being identified as the same caliber cannot be used in any of the pistols; this is justified by its descriptive text claiming the TMP ammo is customized.
Averted in Max Payne, where the only weapons that share ammo are the two 12 gauge shotguns, which are reloaded with loose rounds. Every other ammo type is identified by the weapon it matches; even two of the other guns that do share ammo in real life load it from entirely different magazines that the other could not possibly use.
Subverted, of all things, in Red Faction Armageddon. Every gun takes unique ammunition and picking up a gun will only give ammo if you have a copy of that gun in your inventory. However, the vast majority of your ammunition comes from generic containers loaded with a random amount of ammo for some or all of the guns you have. These containers will only ever contain ammo for the guns in your inventory, so the game (in practice, if not in form) has a sort of universal ammunition pickup.
Wide Open Sandbox
Grand Theft Auto: San Andreas' pistols don't share ammunition (one fires a bigger bullet than the others, and another presumably uses lower-velocity ammo to complement its suppressor). The SMGs and shotguns can take ammo from other weapons in their class.
However, the AK-47 and M4 both share ammo.
Non-video game examples
Most blasters in the Star Wars universe use a substance called Tibanna Gas and Power Packs. Apparently power packs come in a universal format too; the power packs for E-11 rifles and DH-17 pistols are visibly identical (being based on the same real life submachine gun).
KoTOR 2 had you run a across a Proud Warrior Race Guy who was hiding from some predators. If you choose to mock him for managing to run out of ammo, he will ask if you have ever emptied a blaster and needed to reload.
Power packs are separate from the blaster's supply of energetic gas; an E-11 as used by the stormtroopers requires the power pack to be swapped out much like a rifle magazine every 100 shots, while the gas will last for about 1000 shots but require more detailed work and time to refill.
Also worth mentioning is the fact that the above remark hangs a lampshade on the fact that both KOTOR games have Bottomless Magazines.
The Dark Forces Saga had one type of blaster ammo in the first game, and two in later games, both of which could be recharged from the same power stations.
Used incorrectly in Courage Under Fire where a key plot point was whether or not there was ammunition for the M249 SAW. At the same time, there was ammunition available for their various M16s. Both weapons take 5.56mm NATO rounds, and the version of the M249 shown in the film even has an adapter to accept M16 magazines.
A guide to the James Bond movie series states that Bond's favorite gun is the Walther PPK because its ammo is easily found around the world.
Referenced, subverted and parodied in Iron Man 3: Tony asks for Rhodey's extra magazines, and he reminds his friend that "It's a different gun! Those aren't universal!".* While both pistols were firing 9mm ammunition, they used differently designed magazines. The only way for Rhodey to share ammo with Tony would have been to manually remove bullets from one of his magazines and give them to Tony, who would then have to manually reload the magazine, before inserting it into his pistol. Far more trouble than it's worth, and given Tony's aiming skills, just another good reason for those bullets to stay right where they are.
Similar to the Invisible War example in above, in John Scalzi's Old Man's War, the rifles used by the Colonial forces use nanotechnology-based ammo that reconfigures itself into whatever you're asking for—including rockets, grenades, bullets, and microwaves. You can even create your own custom firing macros (eg., fire a grenade followed by a blast of flame for good measure)
Live Action TV
Humorously averted in a scene in Burn Notice where Sam is needling Michael about having no life outside the spy business. Sam offhandedly mentions that his girlfriend Veronica once asked him if bullets came in different sizes.
Being forensics shows, CSI: Crime Scene Investigation and its spin-offs and bandwagon-jumpers routinely avert the trope. In a notable recent example on CSI, the very fact that the rounds used in the crime were of an unusual caliber and design was a plot point. The rounds aren't even commercially available in the States. Turned out they came from a batch of FN P90s stolen from US forces in Pakistan.
Humorously averted in Shameless (US). Frank gets hold of an old WWII Luger and wants to shoot it in order to scare his girlfriend. However, there is no ammunition for the gun so he just grabs some random bullets that fit into the gun. When he tries to fire it, the gun blows up in his hand.
Averted on Elementary. A man murders his wife using a plastic gun that he made using a 3D printer. He later makes another plastic gun from the same blueprints and tries to kill his accomplice with it. However, this time he makes the mistake of using a slightly more powerful bullet and the gun blows up in his hand since the plastic cannot handle the increase in pressure.
The tabletop RPG Shadowrun suggests handling ammo this way: all weapons of the same class use the same ammo, for simplicity.
Then again, its Sega Genesis game fits the above Inexplicable Example table, as Ares Universal clips not only fit any gun they're loaded into (regardless of class), but round themselves off to maintain a roughly equal percentage of ammo remaining across unequal magazine sizes.
Played fairly straight in BattleTech. No matter where or when in the competing Successor States the weapon and ammunition were manufactured, as long as their general type matches it'll fit; AC/5 ammo will work in any AC/5 ever built, never mind that those can canonically come in different calibers. The weapons and feeding systems are even flexible enough to handle special-purpose ammunition that is canonically sufficiently heavier than normal to halve the number of shots! Yet on the other hand, the 'general type' match must be strict — no shooting standard shells from a more advanced (say, LB-X or ultra) autocannon of the same size or vice versa, no feeding missiles from an LRM-15 bin into an LRM-5 rack despite the fact that both get 120 identical missiles to the ton and feeding launchers of the same size from any matching magazine mounted somewhere on your unit is A-OK.
Justified when it comes to Gauss Rifle technology; since they are basically Magnetic Weapons in function, all they need (in-universe anyway) is a sufficiently aerodynamic ferrous slug of adequate weight, and as it is considered high technology, everyone's designs are based on recovered information from a more unified, more advanced era. The Clans, as descendants from the military of this same high-tech era, also drew from the same blueprints. It turns out that the Star League had proved the best results come from flinging a chunk of metal about 125kg at mach 2.
Painfully averted in GURPS: High Tech which gives an exhaustive list of ammunition types. Played more straight in Ultra-Tech where there are only a dozen different rounds between all the guns. High Tech even has a table explaining ammo compatibility (including showing how less powerful and/or smaller parent cartridges fit into their descendant guns, like .38 Special into .357 Magnum, but not vice-versa), and optional rules that let the GM determine the effect of loading incompatible ammo that somehow fits in the gun (with the best result being lower efficiency and possible jamming, and the worst result being the gun exploding).
An odd version in the Wasteland RPG. Ammo calibers were standardized (.45, 9mm, 7.62x39mm, and energy), but the clips themselves were 'universal'—a rifle ammo clip could fit in any rifle. Their capacity was determined only by the weapon they were loaded in: the same clip that put 7 rounds in a .45 pistol gave 30 to a submachine gun.
KULT handles this absurdly, stating that any weapon can fire ammo of the proper caliber or smaller.
At least one expansion splatbook for Mutants & Masterminds / True20 list ammo as Pistol 1-3, Longarm 1-3, and so on, with higher numbers corresponding to higher calibers. So Pistol 1 ammo fits holdout pistols (Derringers), while Pistol 3 ammo would be chambered by heavy pistols like the Desert Eagle.
Most Role Playing Games set in some sort of science fiction genre list only one or two types of ammo for energy weapons, usually an energy cell for 1-handed blasters and a power pack for longarms.
Nerf dartsdo fit with the majority of Nerf guns, it's when you have a non-Nerf brand gun (like Buzz Bee or Air Zone or any knockoff) that you run into problems...
Of course, darts come in a variety of diameters, which obviously won't fit smaller weapons. This is mostly an issue for Bows, the Titan rocket launcher, the Elite Demolisher's underbarrel grenade launcher, and blasters that use either ball ammo or the older Mega Darts.
Modern post-2000s blasters tend to use several common shared ammo types:
All N-strike and N-Strike Elite blasters that use the Clip System magazines share the common Streamline Micro Dart as well as the new Elite Streamline and Elite Suction-cup Darts.
All muzzle-loading blasters and a number of non-CS blasters can share the Sonic Micro/Whistler micro darts or the Suction Cup micro darts. Most of them can also share the Tagger micro darts with Dart Tag blasters.
Several revolver blasters, like the Maverick, Spectre, and Strongarm, are all able to use any Micro Dart due to the short dart posts in their chambers.
All Vortex Blasters share the XLR disc ammo.
Elite Blasters can all use the streamlined Elite Micro Darts, and mostly retain backwards compatibility with the older N-Strike Streamline Micro Darts. The Rebelle and Zombie Strike series blasters also use the same base ammo type. The only exceptions are the Elite MEGA blasters, all of which can use MEGA Elite Darts but not the smaller Elite Micros, some of the Rebelle bows that use unique arrows, some Zombie Strike blasters that are chambered for XLR Disc ammo, and the Elite Demolisher's underbarrel Grenade Launcher, which takes unique "Mega Dart Tech" grenades.
Any type of squirt-gun, of course. Some older ones can even load soda bottles filled with water, in a pinch.
A Fat Albert and the Cosby Kids has an episode where a kid is playing with an unloaded real gun. Eventually, the kids find a real bullet and want the kid to shoot it. However, none of the kids have any idea about calibre specifications for bullets and the gun wielding kid can barely chamber it. Before Fat Albert and the kid's father can get to him, he fires it and the gun explodes in his hand. Fortunately, the father was able to render first aid and get his son to a hospital.
Kind of Truth in Television for armies of the world. The less kinds of ammo you have to stock and distribute, the easier it is to supply your troops. Imagine the logistics nightmare during World War II for the U.S., when any one squad needed .30-06 (M1 Garand, M1903 Springfield, and various machine guns), .45 ACP (M1 Thompson, M3 Grease Gun, and M1911), and/or .30 carbine (M1 Carbine). Compared to today where a squad usually only needs one caliber: 5.56mm.
One of NATO's standardization agreements (4179 specifically) decreed that all NATO weapons chambered for 5.56x45mm were to be designed to take the same magazines as the American M16.note This agreement was never actually ratified, though. Most of the armies who do use those magazines for their weapons tend to go the extra mile and use the licensed Canadian copy of the original gun - other countries like France and Germany do make STANAG-compliant versions of their weapons, but their standard-issue ones use proprietary magazines.
Fabrique Nationale's FNC rifle is designed to use magazines that are nearly-identical to the M16's, so much so that they can be used in the other rifle with no issue (other than that, due to wildly-different actions, the FNC's magazines won't allow the M16's bolt to lock back after firing, so a user can't use the bolt release button to chamber a new round after reloading from empty).
One of the major reasons for the controversial U.S. Army replacement of the Colt M1911 pistol with the Beretta M9 was so that NATO armed forces would only need to stock one type of pistol-caliber ammunition (the M1911 was chambered for .45 while pretty much everyone else was using sidearms chambered for 9mm). This was also what lead to the development of the M14 in the 50's (NATO had standardized the 7.62x51mm round and were pressuring America to adopt a weapon of that caliber, but America wasn't willing to just adopt the FAL like almost everybody else).
As of 2012, the main service pistols of the world's three largest armies fire the 9mm Parabellum. US's M9, China's QSZ-92-9(5.8mm variant was not adopted), and Russia's MP-443 Grach. With the majority of both NATO and SCO's armies using the 9mm, it has become the universal pistol caliber.
The Luger P08 had cemented its cartridge's status as such as early as 1918, with the Bergmann MP18 initially designed to load rounds from its famous 32-round "snail drum" magazines.
Played with in real life. While ammo will work in any gun chambered for it, magazine designs are quite often incompatible. Many civilian gun manufacturers produce proprietary magazines, which would be the only magazines that could work with their firearms. Fortunately, due to the explosion of aftermarket magazines, as well as of the third-party Gun Accessories industry in general, this practice has become much less pervasive over the past 10 years, but especially since the expiration of the Clinton-era AWB in 2004. This phenomenon is partially due to the fact that gun buyers now more frequently go after a generic platform of firearm (such as Kalashnikov, AR-15, M1911...), made by several different companies, rather than buy a brand exclusive gun. Smaller gun companies also tend to make a significant portion of their profits from aftermarket magazine and parts sales, and it is cheaper for such companies to build guns which will work with common magazines for brand name guns (a lot of modern submachine guns are specifically designed to load from Glock magazines, for instance, since there are versions of the Glock in nearly every pistol bullet currently in production). It's pretty much like between buying Ibuprofen or buying Motrin.
Ruger infamously limited its sales of magazines bigger than 5 rounds exclusively to law enforcement. As a result, many small fortunes have been made creating third-party magazines for Ruger firearms. It should be noted that standard magazine capacity for two-handed weapons is anywhere between 10 to 30 rounds, depending on caliber.
The Colt Single Action Army Revolver or "Peacemaker" was available in .44-40 Winchester, which made its ammunition interchangeable with the Winchester Model 1873 lever action rifle. A useful trait in the Wild West, and what started the trend of carrying a carbine and sidearm in the same caliber so that you only had to carry a supply of one cartridge. In fact, the .44-40 was the most popular civilian cartridge for revolvers in general in that era, for exactly that reason. A less common but still popular choice was the .38-40 Winchester, which traded slightly less power for reduced recoil and was also available in both the Winchester 73 rifle and the Single Action Army.
Fabrique Nationale's P90 Personal Defense Weapon and Five-seveN pistol share the same ammo, the 5.7x28mm cartridge, expressly designed for both guns. The AR-57 goes one step further - it is an upper receiver for AR-15 rifles that can be used to make them fire 5.7mm bullets, even loading them from the exact same magazines designed for the P90.
The P90/Five-seveN is the most well-known example (and one of the few surviving ones) of the PDW fad that lasted from about 1985 to 2005, where firearm manufacturers were trying to win government and private security contracts by manufacturing submachine guns that used small, high-velocity rounds that could penetrate lighter body armor and were accompanied by a handgun chambered for the same round. Its biggest competitor, the German H&K MP7 (chambered for a 4.6x30mm round), wound up having its handgun counterpart, the H&K UCP, scrapped after nearly 5 years of delays. The MP7 is a perfect example of a gun that was meant to use this trope and ended up being a near-perfect aversion of it (several other companies now make PDWs that can be special-ordered in or converted to 4.6mm, but it took close to a decade for it to happen).
In general, submachine guns are issued in the service pistol caliber of the nation in question. This is usually 9x19mm Parabellum or .45 ACP.
Generally, while there are different kinds of calibers, many shotguns tend to take 12 gauge shells.
Similarly, there are different sizes of airsoft pellets, but the most common ones are 6mm. Like the above, however, magazines are rarely interchangeable - one could have two or more airsoft versions of the same weapon but be unable to exchange magazines if they're made by different companies, but then load the magazine from one into a completely different pistol made by the same company. Meanwhile, some magazines are stated as compatible but require some form of modification (like wrapping duct tape around them) to get them to fit properly.
One of the main advantages to the .22 Long and its variations was that a rifle fitted for one caliber could take any that had a shorter casingnote this even means that weapons chambered for the long-obsolete .22 Extra Long (discontinued in 1935) can still be fired, as the still-produced .22LR will work just fine with them. A few other calibers (the .38 for instance) also did this. This only works with rimmed cartridges though.
A revolver chambered for .357 Magnumnote dimensions: .379 x 1.59 inches can safely use .38 Specialnote dimensions: .379 x 1.55 inches. The slugs in both are ⌀.357 or +P rounds. They're the same diameter, but the .357 case is slightly longer, so it cannot be used in a .38 weapon for safety reasons. Likewise, the .44 Magnum and .44 Special. As a general rule with revolvers: if it fits in the hole, then it's OK. Automatics firing revolver rounds are generally okay, as well, though in the case of some like the Desert Eagle, you'll have to swap out the springs in its heavy slide to get it to work properly with the lower-powered rounds.
This sort of thinking was the reason why nearly all Soviet small arms from at least 1891 to about 1951 fired 7.62mm bullets - even though they had different such cartridgesnote 7.62x54mmR for rifles and machine guns, 7.62x25mm for pistols and SMGs, and later 7.62x39mm for assault rifles, if they ran critically short of barrels for anything during wartime production, they could take old Tsarist-era Mosin-Nagant rifles and recycle their barrels for use in other guns. For example, one of the old 31.5 inch barrels from the original Mosin-Nagant long rifle could be cut down into 3 PPS-43 submachine gun barrels. And early production PPSh-41 SMGs were made by taking 29 inch barrels originally intended for the M91/30 Mosin-Nagant and cutting them in half, then trimming the halves down to the proper length.
AK-pattern rifles may have honorary status for this trope; in addition to the original 7.62 and 5.45mm bullets for the AK-47/AKM and AK-74, the weapon's action has also been adapted or copied for 5.56x45mm, 7.62x51mm, and even 12- and 20-gauge weapons, among various other calibers. At one point during the War in Afghanistan, SOCOM even put out an order for an M4 variant firing 7.62mm Soviet because that sort of ammo was very plentiful on the bodies of insurgents killed in combat.
A word of warning: Some rounds are available with more powerful propellant loads. It is entirely possible to load up a .38 Special revolver with overloaded .38 Special ammunition that will blow the gun apart (in your hand, no less) if the particular gun is not designed to handle that propellant load. This is most common with older 9mm handguns (modern 9mm cartridges, particularly new Russian overpressure ones, are more powerful than they had been a hundred years ago due to improved gun metallurgy) and rifles in .223 Remington or .308 Winchester (the military 5.56 and 7.62mm NATO rounds have the exact same dimensions, but 5.56mm has a higher propellant load than .223, while 7.62mm has a lower load than .308; a gun designed for the higher-powered round can fire the other with no trouble, but the reverse is not true).note As an interesting tidbit, the original 9mm handgun, the Luger, had gained a reputation among US soldiers who had stolen them from dead Germans during World War II as being unreliable due to the opposite problem - American ammo companies had produced 9mm bullets with less-powerful loads than normal in an attempt to prevent the soldiers from accidentally blowing up their war trophies, but in most cases the rounds weren't even powerful enough to properly work its mechanism.
And to go even further, some guns require ammunition that might as well be proprietary to function properly, even if they are the exact same dimensions. The French FAMAS fires the 5.56x45mm, but it cannot fire the standard NATO round. The rifling cannot stabilize modern ammunition, and the powerful blowback action will tear apart standard brass cartridges. But hey! At least the G-2 variant uses STANAG magazines.
This was extremely common in the age of smoothbore muzzle-loaders, although if calibres were markedly different it could make a difference, so during the Napoleonic Wars it was possible to shoot French musketballs from British "Brown Bess" muskets, but balls suited to the Bess's larger calibre could not be fired from the standard French "Charleville" musket.
Wellington ordered that only pistols that could fire musketballs could be used in his army. This however had an underlying reason: Pistols of a smaller calibre usually were dueling pistols.
In the cases where a bullet wouldn't fit as-is, since it was just a bare lead ball, one could have captured enemy ammo, melted down the lead (which melts at a relatively low 327.46 °C/621.43 °F), and recast it in the appropriate caliber.
Intentionally subverted by the Soviets. Before and during World War II, the standard size for a medium mortar was 81mm. The Soviet Army built a medium mortar firing an 82mm round. That wasn't just cheap oneupmanship. The 82mm mortar could fire captured enemy ammunition at reduced efficiency, but if the enemy captured a supply of 82mm ammunition, it was completely useless to them unless they captured a mortar as well.
They did so again when they adopted the 9x18mm pistol round in the 50's - under normal circumstances, a round of that diameter could theoretically be loaded into and fired from a weapon designed for the NATO standard 9x19mm, so the 9mm Makarov was designed to use a slightly-larger 9.2mm bullet. This may have been a result of the earlier 7.62x25mm Tokarev cartridge being dimensionally identical to the German 7.63x25mm cartridge as used in the famous C96 pistol - German units that had captured Soviet submachine guns were able to load 7.63mm Mauser into the guns that weren't converted to 9mm and fire them without incident.
The Steyr AUG assault rifle also comes in a "Para" version firing 9x19mm. Conversion kits exist to change 5.56mm AUGs to 9mm. Counts doubly as this trope, as this version of the AUG is designed to load from the same magazines as Steyr's earlier MPi69 submachine gun. And, as above about NATO standardization agreements, there is a version of the gun (AUG NATO) designed to load from M16 magazines, though at the cost of ambidextrous usability.
One weird example is the Destroyer Carbine, chambered for the 9x23mm Largo pistol cartridge. This carbine was developed in Spain for police use. It was a bolt-action weapon whose action was basically that of a scaled-down Mauser 1893 rifle (not the WWI Mauser Gewehr 98 or the K98 we usually see, as the 1893 Mauser bolt cocked upon closing, whereas the G98 and K98 bolts cocked on opening) and fed by the six-round or ten-round magazines used by the Bergmann-Bayard Pistols employed by said Spanish Police units. So the average Spanish cop after 1921 was usually armed with both a pistol and a carbine chambered for the same ammo. In 1976, the Destroyer was no longer produced, having been slowly replaced by the Star Z-62, also chambered for 9x23mm Largo. Picking on cops over there tends to be Bullying The Dragon.
This is even the case for bows and arrows, which have to be precisely matched to each other's draw length, stiffness etc. if they're to be used with anything approaching accuracy. Some armies in the past used bows with extremely short draw lengths, so that their short arrows would be literally useless to the enemy.
Of special note is the British Sten, a submachine gun that was not only chambered in the same 9mm Parabellum that was standard issue in the German army, but was designed specifically to use the same magazines as the German MP-38 and MP-40 submachine guns. Likewise, the Besa machine gun (used only in certain tanks) was left in its original 7.92x57 Mauser chambering due in part to the time and effort needed to convert it to .303, the fact that Armoured Corps' supply chain was separate from the rest of the British Army and so would not be handicapped by using a nonstandard round, and the ease of using captured enemy ammunition.