History Main / UniversalAmmunition

* The ''VideoGame/SniperElite'' series applies this inconsistently: while each individual weapon has its own individual ammo pool, save for running over a dropped weapon while you already have another of that weapon on you, ammo pickups are for universal based on what slot a weapon goes in, no matter how unlikely. A German soldier with an MP 40 may have a ''very'' small handful of 9x19mm bullets as used in that MP 40 - and depending on each game's preferences it's a toss-up whether what bullets he does have will actually be usable in an MP 40 or only in the Luger or Welrod pistol - while it's far more likely for him to have a dozen or so rifle bullets in anything from American .30-06 to Japanese 7.7x58mm.

* ''VideoGame/SniperElite'' series:
** Inconsistently applied in ''VideoGame/SniperEliteV2'', 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.
** ''VideoGame/SniperElite3'' uses the same mechanics, though with the addition of loose ammo pickups that replenish weapons based on their class, looting rules changed to prioritize SMG ammo over handguns, and the ability to grab ammo from copies of the same weapon dropped by enemies.
** ''VideoGame/SniperElite5:'' All rifles use Rifle Ammo, all submachine guns use SMG Ammo, all pistols use Pistol Ammo. No explanation is given.

** Similar ideas have also fueled the creation of some more modern rounds, like the 6.5x25mm CBJ or .300 AAC Blackout, which are designed to be as compatible as possible with their parent cartridges, respectively 9mm Para and 5.56mm NATO. The new rounds are essentially the existing casings simply necked differently to take a different-diameter bullet, requiring nothing more than a barrel change to convert an existing 9mm or 5.56mm weapon to 6.5 or .300 - the actual casing is the same size, so they will still fit in the same magazines and still work with the same bolts. This goes back at least to 1994 with the design of the .357 SIG cartridge, which was based on the .40 S&W, to the point that they have the same base diameter and almost the same overall length (.357 SIG is only about a tenth of a millimeter longer), so all that's absolutely necessary to convert between the two is a barrel change - however, quick caliber conversions were not one of the design priorities, so switching a .40 S&W gun to .357 SIG may also require a stronger recoil spring to handle the .357's higher operating pressures. In turn, the .40 S&W is based off of 10mm Auto, but with a case shortened by about three and a half millimeters, about like the difference between a Magnum round and its Special counterpart - one can technically fire .40 S&W through a 10mm Auto pistol, and if it does work it can even be safer and more convenient (lower pressure generated for less wear on the gun, less recoil for a more comfortable experience - though with the caveat that it increases the risk of the gun failing to cycle - and cheaper ammo to keep it fed), but 10mm Auto through a .40 S&W gun simply won't work without more extensive modification, since the 10mm Auto won't fit in the smaller gun,[[note]]even [[https://www.youtube.com/watch?v=wzh16zi55G0 this video]] of a [=USP40=] converted to 10mm notes that, among other modifications, it required modifying a metal mag from a different 10mm gun - the plastic walls of the existing [=USP40=] mags are too thick to fit the longer 10mm Auto - and that the end result still can't eject an unfired 10mm cartridge through its ejection port, requiring the user to remove the magazine and push an unfired round down through the magazine well[[/note]] and would probably damage the gun even if it did (part of the reason .40 came into existence was because the shorter case length allowed designers to easily convert their existing 9mm frames for the slightly larger-caliber bullet, which means even a lot of handguns in .40 S&W were more easily damaged by firing .40 through them).

* Inconsistently applied in ''VideoGame/SniperEliteV2'', 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. ''VideoGame/SniperElite3'' uses the same mechanics, though with the addition of loose ammo pickups that replenish weapons based on their class, looting rules changed to prioritize SMG ammo over handguns, and the ability to grab ammo from copies of the same weapon dropped by enemies.

* 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 casing. 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, such as the .38 or the aforementioned 12-gauge, also did this. This only works with rimmed cartridges though, with ''very'' rare exceptions.[[note]]e.g. Smith & Wesson's variation of the M1917 revolver, to supplement the M1911 pistol in UsefulNotes/WorldWarI, was specifically designed to be able to take regular .45 ACP cartridges without use of a moon clip, while Colt's version of the same weapon required either a moon clip to hold .45 ACP, or a special .45 Auto Rim version of the ammo; though this in turn meant reloading S&W's version with regular .45 ammo required shaking the casings loose manually, since there's no rim or clip for the ejector to catch onto.[[/note]]

** 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 9x18mm, 9x19mm, 5.56x45mm, 7.62x51mm, and even 12- and 20-gauge, among various other one-off calibers for specific military purposes. The modern AK-12, around 2013, was being advertised for its adaptability, able to swap between various Russian and NATO calibers (including most of the listed ones just above) with little more than a barrel and magazine change; as of it entering production, this doesn't appear to be the case, though there are still separate variants of the weapon for different calibers as before (the AK-12 in 5.45mm, an AK-15 in 7.62mm Soviet, and an AK-19 and AK-308 for foreign customers in respectively 5.56mm and .308 Winchester).

** 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 due to poor ballistics. 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). The fact that the [=MP7=] is itself basically a pistol (albeit [[HandCannon a very large one]]) that can be holstered on a soldier's belt and can easily be fired one-handed when in semi-auto mode helped with H&K deeming the UCP to be redundant.

* 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 casing[[note]]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[[/note]]. A few other calibers, such as the .38 or the aforementioned 12-gauge, also did this. This only works with rimmed cartridges though, with ''very'' rare exceptions.[[note]]e.g. Smith & Wesson's variation of the M1917 revolver, to supplement the M1911 pistol in UsefulNotes/WorldWarI, was specifically designed to be able to take regular .45 ACP cartridges without use of a moon clip, while Colt's version of the same weapon required either a moon clip to hold .45 ACP, or a special .45 Auto Rim version of the ammo; though this in turn meant reloading S&W's version with regular .45 ammo required shaking the casings loose manually, since there's no rim or clip for the ejector to catch onto.[[/note]]
** A revolver chambered for .357 Magnum[[note]]dimensions: .379 x 1.59 inches[[/note]] can safely use .38 Special[[note]]dimensions: .379 x 1.55 inches. The slugs in both are ⌀.357[[/note]] 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. And the .45 Long Colt and its lengthened and more powerful brothers, the .454 Casull and .460 S&W Magnum. Some modern revolvers are also sold with a second cylinder meant for an automatic pistol cartridge of the same diameter, e.g. a .357 Magnum revolver (9x33mmR) coming with a cylinder for 9x19mm. 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 there are more factors to take into consideration, including the strength of the recoil springs (generally requiring swapping between weaker and stronger springs for respectively Special and Magnum loads; stronger means weaker cartridges won't be able to push the slide back far enough to cycle properly, while weaker means full-power cartridges can outright damage the gun).

* Forgotten Weapons goes into detail on the [[https://www.youtube.com/watch?v=szCW_52zPIk many different kinds of 9mm]] bullets, and the compatibility and legal difficulties that can arise from this diversity of ammo types all using similar dimensions.

These cartridges are then loaded into clips and magazines[[note]]clips feed magazines and magazines feed guns. Clips were the most common way to load guns in the days of everything having an integral magazine, but nowadays they're rather rare since most everything has a detachable magazine; if you are asking for a box of bullets that you will shove up the handle of a handgun, you want a magazine[[/note]] 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 AcceptableBreakFromReality. This trope is similar to OneBulletClips since both of them treat ammunition like water that's just decanted into a container of the right type when needed; here the container in question and its contents can magically change depending on what it is being attached to, whereas there it's more about the way every magazine not being held is immersed in the ammo-liquid until it is as full as possible.

** Glock magazines are particularly noteworthy for this - not only can different versions of it in one caliber exchange magazines, within reason (there's a minimum size of magazine any one Glock can take depending on frame size; a Glock 26 can take the 17-round magazines of the Glock 17, but the 17 wouldn't be able to load the shorter 10-rounders of the Glock 26), but the sheer ubiquity of the Glock (including there being versions of it, and a wide variety of magazine sizes for it, in just about every pistol cartridge currently produced) means that several modern submachine guns and pistol-caliber carbines are also designed to take Glock mags, such as the Kriss Vector and the Ruger PC Carbine. Since Glock magazines already exist for most popular pistol calibers and are known to work very well, it saves time and money on designing a weapon to just use them instead of designing your own mag. And since so many shooters already own a Glock, they'll likely appreciate a carbine that uses the same mags as their pistol. Starting in the latter half of the 2010s, a veritable cottage industry has sprung up for AR-15 variants that are designed to fire pistol calibers using Glock mags such as the Just Right Carbine, the Palmetto State Armory AR-9, and the TNW Aero Survival Rifle. One thing to note is that many pistol magazines actually do have very similar dimensions, usually with the only major difference being where the cutout for their parent weapon's magazine release to interface with them is, so in a pinch it can be possible to fire a gun with a magazine meant for a completely different gun so long as you hold it in place yourself.

** Ruger infamously limited its sales of magazines bigger than 5 rounds exclusively to law enforcement in the 1990s after a terrible school shooting in Stockton, CA in 1989. 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, and even state laws restricting the maximum capacity of a weapon in civilian hands never limit it to less than 10. The Ruger company abandoned these restrictions in the mid-2000s after Bill Ruger died in 2002 (he had been the one to personally halt the sales of >5-round magazines) and the Bill Clinton-era Assault Weapons Ban expired in September 2004.

** One of the ways in which Heckler & Koch's UMP CoolGuns/{{submachine gun|s}} was meant to be a modern upgrade to the [=MP5=] was that it was expressly designed to be easily converted from one caliber to another (as opposed to the [=MP5=] being chiefly a 9mm weapon, with alternate calibers requiring entirely separate variants like the [=MP5/10=]) - all that's required is a swap of the bolt, barrel, and magazine, the mags being designed to all fit into one universal magazine well.

* ''VideoGame/SeriousSam'''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 [[RareGuns 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 ammo[[note]]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[[/note]].

* The [[RareGuns/BattleRifles MAS-49 and MAS-49/56]] have a unique kind of magazine system, where the magazine catch lever is on the magazine itself rather than part of the rifle.[[note]]It was part of a production shortcut to get the earlier MAS-44 out as quickly as possible in the last days of UsefulNotes/WorldWarII; it still wasn't fast enough to see service, but by the time they made the MAS-49 they kept using the same mag catch, since they already had thousands of the appropriate magazines.[[/note]] This allows owners of surplus rifles to easily convert magazines for other weapons into extended MAS-49 magazines, assuming they fit into the 49's mag well and hold the appropriate ammo (like a 25-round FM 24/29 magazine for a 7.5mm version, or a 20-round FAL magazine for one of the rare 7.62mm NATO conversions), by welding a spare catch onto them.

** 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 9x18mm, 9x19mm, 5.56x45mm, 7.62x51mm, and even 12- and 20-gauge, among various other one-off calibers for specific military purposes. The modern [[RareGuns/AssaultRifles AK-12]], around 2013, was being advertised for its adaptability, able to swap between various Russian and NATO calibers (including most of the listed ones just above) with little more than a barrel and magazine change; as of it entering production, this doesn't appear to be the case, though there are still separate variants of the weapon for different calibers as before (the AK-12 in 5.45mm, an AK-15 in 7.62mm Soviet, and an AK-19 and AK-308 for foreign customers in respectively 5.56mm and .308 Winchester).

* Back to ''VideoGame/HalfLife1'': 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 contents of said battery packs.

* A guide to the ''Film/JamesBond'' movie series states that Bond's favorite gun is the Walther PPK because its ammo is easily found around the world, fitting his globetrotting lifestyle. This is probably no longer canon post-reboot, when ''Film/{{Skyfall}}'' explicitly stated that the new, high-tech PPK he'd been issued was in 9mm Short/.380 ACP rather than 7.65mm/.32 ACP, which dates back to the tail end of the 19th century and was so ubiquitous as a police and self-defence pistol calibre that it was grandfathered in behind the Iron Curtain. However, China still uses it in a limited capacity for police sidearms and .32 rounds are still fairly popular.

* Averted in ''VideoGame/{{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. The game does occasionally feature unique versions of guns chambered for a different ammo type or, in later games, the ability to convert a gun chambering Warsaw Pact ammo to use the equivalent NATO caliber (and vice versa) by visiting a tech.

** ''VideoGame/ReturnToCastleWolfenstein'' has more ammo types, but is still careful to only let those guns that actually used them share - the returning Luger and [=MP40=] all use 9mm Parabellum rounds, as does the British Sten, while also adding .45 ACP rounds for the Colt 1911 and Thompson SMG, and 7.92mm Mauser rounds for the Mauser rifle and FG 42. Other guns, like the Panzerfaust, flamethrower, tesla cannon and Snooper Rifle, each have their own ammo types, and you're not going to be pilfering more for them from the corpses of your enemies unless you stole the gun from one in the first place. Interestingly, the Venom gun has its own unique ammo supply, which is justified but in [[GameplayAndStorySegregation two entirely-incompatible ways]] - the in-game operations manual you can find before you get it states that it uses a specific variety of 7.92mm ammo that the Mauser and FG 42 presumably are not using, while its ammo pickups are identified as [[{{BFG}} 12.7mm (i.e. .50-caliber)]] when you grab them.

* Since the weapons in ''VideoGame/{{Splatoon}}'' use ink rather than physical bullets, every weapon can be reloaded by just shifting into squid/octopus form and swimming through the player's ink color.

** ''VideoGame/ResidentEvil4'' again may fall into this. Among the weapons that share ammo are the Broken Butterfly revolver and the semi-auto VideoGame/{{Killer7}}, both of which take .45-caliber rounds. So long as they were rimmed cartridges or at least attached to a moon clip, this would be entirely possible; revolvers will generally shoot anything you can fit into their cylinders. Also, the two scoped rifles, the Springfield M1903 and the H&K [=SL8=], share ammo despite the fact that the real weapons are designed for respectively .30-06 Springfield and .223 Remington; however, if you read the inventory description for the Springfield, it does flat-out state the gun in question has been rechambered for .223.

** The French did a similar subversion with the Chatellerault M1924/29 a few decades prior, though for the opposite reason as the Soviets. When it was first adopted in 1924, it was designed to use a 7.5x57mm round. This round was close enough in appearance to the German 7.92x57mm Mauser round that some auxiliary troops in Morocco who used captured Mauser rifles and ammo would accidentally load Mauser rounds into an M1924/29, [[ExplosiveOverclocking with disastrous results]]. Starting from 1929 it was modified to use a slightly-shortened and now much more famous 7.5x54mm round so that 8mm Mauser rounds wouldn't fit into it or any future French weapons. This also led to a subversion decades later on the surplus market, since up until the 5.56mm FAMAS, all French military rifles afterward were standardized on the 7.5x54mm round: ammo for Swiss 7.5x55mm rifles is extremely common on the surplus market, and both cheaper and of considerably higher quality than the surplus French 7.5x54mm ammo (which is usually Syrian surplus, with the French Army having actually used up most of their own 7.5x54mm)... but the case is ''just'' long enough that they're not interchangeable, even though the French round is directly derived from the Swiss one.

** Once upon a time, picking up ''any'' weapon dropped by a dead player gave you half of your maximum ammunition, even if it was something like the Pyro's flamethrower, the Soldier's rocket launcher, the Demoman's bottle, a wooden bat, 150kg of metal in the shape of a minigun, or a ''dead fish''. Spies could also somehow recharge their cloak meter with these same objects, as could the Engineer regain metal to build sentry guns and teleporters (where this got odd is that dropped [[NiceHat hats]] gave no metal, despite them [[ItMakesSenseInContext being made out of enough of the stuff]] to craft at least ''36'' weapons). With the Gun Mettle update, a dead player will drop both the weapon ''and'' a medium ammo pack. The dropped weapon instead can be [[TheEnemyWeaponsAreBetter freely picked up and used by another player of the respective class]] until their death or the end of the match.

** ''VideoGame/ResidentEvil2Remake'' fixes leftover issues of this nature from the original game. Claire now gets two revolvers, her first one being a Smith & Wesson Bodyguard in .38 Special that actually could, at least in theory, take 9mm Para rounds like Leon's [=VP70=]. Her second, replacing the SAA as her bonus weapon in a B scenario, is a Ruger Blackhawk, which gets its own unique ammo type that is only shared with Leon's equivalent bonus weapon, an M1911.

* Taken UpToEleven in ''VideoGame/DigitalDevilSaga'', where the same ammunition can be used in not only handguns and rifles, but shotguns and ''grenade launchers''.

* ''VideoGame/Left4Dead'' justifies this: all guns have separate ammo pools and replenish that or get upgraded ammo from the same pile/box, but they are all clearly shown to contain a variety of different types of ammo. There are also a few weapons that cannot be reloaded from the ammo piles but can get explosive or incendiary ammo, specifically [[{{BFG}} the grenade launcher and M60]].
* ''VideoGame/MetroidPrimeHunters'' 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.

* ''VideoGame/{{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.

** ''VideoGame/ResidentEvil4'' again may fall into this. Among the weapons that share ammo are the Broken Butterfly revolver and the semi-auto VideoGame/{{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. Also, the two scoped rifles, the Springfield M1903 and the H&K [=SL8=], share ammo despite the fact that the real weapons are designed for respectively .30-06 Springfield and .223 Remington; however, if you read the inventory description for the Springfield, it does flat-out state the gun in question has been rechambered for .223.

** It seems like simplifying (as best they can anyway) logistical needs has been the main US doctrine since metallic cartridges became a thing. For instance, when the US Army was having trials for a semi-automatic rifle, the Pedersen and Garand rifles initially used a .276 caliber bullet. However, UsefulNotes/DouglasMacArthur, Army Chief of Staff at the time, rejected the round, especially after .30-06 version of the Garand rifle was known. Another point of this is speculation of why the US didn't try to create a weapon to counter the Soviet's "stealth" VSS Vintorez sniper rifle: the Vintorez needed subsonic ammunition with an effective range of 300m. To achieve that, the Soviets needed a new cartridge and the US didn't want to create a special purpose weapon with specific ammunition (not to mention this sort of thing kept failing miserably for other "specialized" items the US Army tried out).

** Modern H&K pistols have similar compatibility, though only within the specific niche of H&K pistols. Basically, if a magazine fits into a [=VP9=], then it will also fit into a P30, P2000, or USP Compact of the same caliber, and likewise magazines for .45 version of the USP Compact work with the [=HK45=] - however, magazines made specifically for the P2000 and USP Compact simply won't fit into the [=VP9=] or P30 because of the shorter size (both being concealed-carry pistols with shorter grips), and it's also impossible to exchange magazines between any of these pistols and any full-size version of the USP because of the different widths (the USP magazines are too wide to even get into the other pistols, while the other magazines are too thin to lock properly into the USP).

* Averted in ''VideoGame/MaxPayne'', 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 (the 9mm Beretta 92s and M11s) load it from entirely different magazines that could not possibly interchange.

** Most ridiculously, in ''Far Cry 2'' "flame" ammo is shown as a small gas can, which can somehow morph into magnesium flares for a flaregun.

* 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).

* Of special note is the British Sten, a submachine gun that chambered in the same 9mm Parabellum that was standard issue in the German army, since the ammo and weapons firing it were far easier to get their hands on (early battles in the African theatre netted them several thousands of 9mm rounds from surrendering Italian soldiers) than replacing the .45 Thompsons America lend-leased to them for the early stages of the war. Its magazines were even a direct copy of those of the MP 38, to facilitate using enemy ammo. Note though that there were minor differences between the MP 38 mags (and the clones made for the Sten) and the MP 40 mags that made them not interchangeable, despite what many have been lead to believe; at best, a Sten mag will technically fit into the magwell for an MP 40, but would require modification to either the gun or the magazine to hold properly.

** One of the ways in which Heckler & Koch's UMP CoolGuns/{{submachine gun|s}} was meant to be a modern upgrade to the [=MP5=] was that it was expressly designed to be easily converted from one caliber to another (as opposed to the [=MP5=] being chiefly a 9mm weapon, with alternate calibers requiring entirely separate variants like the [=MP5/10=]) - all that's required is a swap of the bolt, barrel, and magazine, the mags being designed to all fit into one universal magwell.

** Similar ideas have also fueled the creation of some more modern rounds, like the 6.5x25mm CBJ or .300 AAC Blackout, which are designed to be as compatible as possible with their parent cartridges, respectively 9mm Para and 5.56mm NATO. The new rounds are essentially the existing casings simply necked differently to take a different-diameter bullet, requiring nothing more than a barrel change to convert an existing 9mm or 5.56mm weapon to 6.5 or .300 - the actual casing is the same size, so they will still fit in the same magazines and still work with the same bolts. This goes back at least to 1994 with the design of the .357 SIG cartridge, which was based on the .40 S&W, to the point that they have the same base diameter and almost the same overall length (.357 SIG is only about a tenth of a millimeter longer), so all that's absolutely necessary to convert between the two is a barrel change - however, quick caliber conversions were not one of the design priorities, so switching a .40 S&W gun to .357 SIG may also require a stronger recoil spring to handle the .357's higher operating pressures. In turn, the .40 S&W is based off of 10mm Auto, but with a case shortened by about three and a half millimeters, about like the difference between a Magnum round and its Special counterpart - one can technically fire .40 S&W through a 10mm Auto pistol, and if it does work it can even be safer and more convenient (lower pressure generated for less wear on the gun, less recoil for a more comfortable experience - though with the caveat that it increases the risk of the gun failing to cycle - and cheaper ammo to keep it fed), but 10mm Auto through a .40 S&W gun simply won't work without more extensive modification, since the 10mm Auto won't fit in the smaller gun,[[note]]even [[https://www.youtube.com/watch?v=wzh16zi55G0 this video]] of a [=USP40=] converted to 10mm notes that, among other modifications, it required modifying a metal mag from a different 10mm gun - the plastic walls of the existing [=USP40=] mags are too thick to fit the longer 10mm Auto - and that the end result still can't eject an unfired 10mm cartridge through its ejection port, requiring the user to remove the magazine and push an unfired round down through the magwell[[/note]] and would probably damage the gun even if it did (part of the reason .40 came into existence was because the shorter case length allowed designers to easily convert their existing 9mm frames for the slightly larger-caliber bullet, which means even a lot of handguns in .40 S&W were more easily damaged by firing .40 through them).

** For example, the American [=M16A2=] and the French FAMAS F1 are both chambered in 5.56x45mm. However, the FAMAS has a 1:12 rifling twist rate, which is designed to be used with the M193 55-grain cartridge (also known by the commercial name .223 Remington), while the [=M16A2=] has a 1:7 twist rate optimized for use with the heavier [=SS109=]/M855 62-grain cartridge; moreover, the FAMAS' operation (a stronger lever-delayed blowback system) means standard brass casings are liable to rupture when used in the weapon, requiring steel-cased rounds for the best results. The FAMAS ''can'' properly chamber and fire the newer 62-grain round, but the rifling will not stabilize it properly, resulting in poor accuracy. The G2 variant of the FAMAS has a 1:9 twist rate, which could properly stabilize both 55 and 62-grain rounds, but was not widely adopted. The NATO standard [=SS109=]/M855 also has higher operating pressure than M193/.223 Remington, so it can be dangerous to fire in some rifles chambered for the earlier version. While most military rifles don't have this issue (the FAMAS for example is a very robust rifle that can handle pressures well in excess of either loading), this isn't always the case and care should be exercised in particular with civilian sporting rifles chambered for .223 Remington.

* 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 17.5mm musketballs from British "Brown Bess" muskets, but 18mm 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 musket balls could be used in his army. This however had an underlying reason: pistols of a smaller calibre usually were dueling pistols.

* Intentionally subverted by the Soviets. Before and during UsefulNotes/WorldWarII, 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.

* Grid-type vertical launch systems are the closest thing to this trope for naval missiles - the American Mark 41, the most common type, can fire seven different missiles and replaced no less than six different missile launchers. Unfortunately, there is no trans-national compatibility, something most clearly shown with the [[UsefulNotes/SouthKoreansWithMarines South Korean Navy]], which on its larger ships operates the Mark 41 for American standard missiles and an indigenous K-VLS for locally-built missiles.

* Discussed in The Chieftain's Hatch when [[https://www.youtube.com/watch?v=juluyzxVSZY covering the M3 Medium tank]]. The initial ammunition used, both high explosive and armor piercing, for the 75mm cannon had issues. The British, who was a prime user of the tank, solved the issues by using another country's ammunition of the same caliber. For the high explosive round, they found the French's 75mm version had a better fuse, and since the original ammo was based off of it, it was a matter of swapping the fuses. For the armor piercing round, they captured a bunch ammo made for Germany's 7.5cm cannon and it happened to fit in an American cartridge with only minor modifications.

* Most ''VideoGame/GrandTheftAuto'' games play this trope straight.