An arm cannon sounds great until you need to button your shirt.
— Atomic Robo points out the flaws in this type of equipment
A specific type of Artificial Limbs that's Exactly What It Says on the Tin: a ranged weapon integrated with a character's arm.
There are two general types. One is the complete replacement of a hand, or even an entire arm, with a gun. Realistically, these make very little sense, as getting an Artificial Limb would allow you to hold a gun, and do other mundane tasks with the limb when you're not in combat. Arm Cannons that fully replace the limb are also prone to suffering Fake Arm Disarms, as they can be destroyed without gravely injuring the wielder. A specialised version is a robotic hand that turns into a gun. This version often overlaps with the Hand Blast trope.
The other, more mundane version, is simply a gun that's attached to the arm - generally with the hand still intact. This might also involve Artificial Limbs, but just as often uses a special harness or weaponised vambrace. Occasionally, the weapon may fit over the forearm and hand, giving the appearance of the first kind. More discreet versions often overlap with Nothing Up My Sleeve. Aiming it is usually a simple matter of pointing in the general direction of whatever it is you want to shoot. Using your free arm to brace the cannon before firing is optional, partly because reducing weapon sway and/or recoil improves your accuracy, but mostly because it makes the resulting shot look more powerful (or at least more awesome).
Both kinds generally come with Bottomless Magazines, and are often fired by thought alone. Larger versions of both types are common on Humongous Mecha, which can be conveniently designed to carry them from the start.
For a weaponised arm that isn't a gun, see Power Fist, Rocket Punch, and especially Blade Below the Shoulder (and sometimes Pile Bunker). Not to be confused with Hand Cannon. Or, for that matter, Armed with Canon. Compare Shoulder Cannon, Leg Cannon, and Hand Blast. Finger Firearms is a small subtrope.
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First Kind (arm is cannon)
Anime & Manga
Dragon Ball Z: Android 16 had two cannons called "Hell Blasters" in his upper arms. In order to use them, he had to remove his forearms, usually holding them in his armpits. It helps that his forearms can also be used for a Rocket Punch.
In Dragon Ball, villain Tao Pai Pai also got an Arm Cannon after being cybernetized.
004/Albert Heidrich from Cyborg 009 looks normal, but as part of his cyborgization process all his limbs were replaced and now contain variable quantities of armament. The most remarkable are his finger guns (take that literally) and the rocket-launcher in his knees.
Not to mention, in older continuities he actually has a nuclear bomb stored in his stomach. Which he's willing to use in a suicidal attack in The Movie. He lives to tell, though.
Astro Boy in the 2003 series. Before that, he had rocket hands, but rarely used them, as his weapon of choice was instead a pair of machine guns that popped out of his tush.
Remake Pluto also has Gesicht, whose hands turn into cannons (one shoots non-lethal gas, the other shoots a powerful shell). Another robot, North Mk. II, cheats a little by having extra limbs devoted to his weapons.
Mazinger Z, other than shooting its fists, hid a cannon in each upper arm shot drilling missiles. Several Mechanical Beasts had cannons (or even machine guns like Balkan P5) instead of limbs. An example stands out is Debira X1 (a beast seemed a blend between bat and vampire) had a hidden leg cannon (and blew one whole Mazinger's arm off with it).
Combattler V: Combattler-V is the king os this trope among Humongous Mecha. He is equipped with Atomic Burner (its hands withdraw inside its arms and are replaced with a flamethrower), Rock Fighter (it shoots missiles from its fingers) and Battle Guleggar (Combattler releases a chain with metal collar from the arms, used for grabbing opponents by the neck).
Voltes V: Voltes Bazooka. The entire weapon is hidden in his right arm, and is deployed by tilting the right hand downward (which also becomes the weapon's grip held by the left hand) allowing the barrel to telescope out of the arm.
Daimos: Daimos has the Daimos Chain Shark, two chains with a hook shoot from above Daimos' hands. They can be used to deal damage or to drag the enemy.
Hyūgo Kujiranami in Rurouni Kenshin first uses an Armstrong Cannon for an arm, and then a Grenade Launcher in his plot to get revenge on Kenshin for cutting off his arm and not killing him as a warrior should.
Kazuhiko, the main character of the CLAMP manga Clover, has a prosthetic arm that can transform into a weapon.
Vash the Stampede in Trigun has something similar to this: his Artificial Limb folds out into first a pistol, then later a submachine gun. By tapping into the plant power inside his revolver, each of his arms can also morph into something akin to a Wave Motion Gun.
Not very common in Ghost in the Shell: Stand Alone Complex for most of the logical reasons. One of the few exceptions is a political assassin who has a shotgun built into her forearm. She cocks it by pulling back her own thumb, and it's modified to shoot a roll of gold coins into the target — the target being a wealthy plutocrat.
There is a second exception: in Innocence, Batou reveals that he has a cannon (well, a single-shot large-caliber firearm) built into his arm.
Similar to Batou's built-in arm cannon, a Russian KGB agent from the anime series sports a large-bore cannon built into her elbow. It is presumably a last-ditch weapon, as it is rather cumbersome to fire — her forearm must fold backward on a hinge to expose the muzzle.
Angelfeathers had a shotgun built into his wrist.
Motoko Aramaki built a grenade launcher into her own arm.
The title hero of Space Adventure Cobra has a beam weapon replacing his left arm, the "Psychogun", which fires beams powered by his psychic energy. When not in use, he covers it with a prosthetic arm and hand.
Guts in Berserk has a Steam Punkmechanical left hand with a gunpowder cannon under it, the original having been chiseled off with a broken sword by Guts himself after it was caught in a demon's jaws during the Eclipse. To work it, he has to flip the hand up and pull out a lever buried in the side of the cannon. The arm also has a repeating crossbow built into it, which he uses for killing human foes and torturing fallen demons.
After losing an arm (and an eye and a leg) in a No One Could Survive That moment in Gundam SEED, Andrew Waltfeld appears in the sequel series Gundam SEED Destiny sporting a prosthetic replacement which, in a pinch, turns out to have a gun built into the wrist, revealed by partly detaching the hand.
RizeGreymon in Digimon Savers had a huge revolver arm transformed from his Rookie form's wrist straps.
Omnimon has an arm cannon too, naturally from the MetalGarurumon side of him. It's utilized for his attack/technique called Supreme Cannon (known as Garuru Cannon in Japanese).
The protagonists of Ayakashi and Mushi-Uta have organic arm cannons. For the former, he normally has a Power Fist, but in the finale, his arm mutated into a dragon cannon. For the latter, his Bond Creature attached to his gun, and as the influence from the merger spread, it mutated his arm and, near the end, most of his body.
In an episode of Fullmetal Alchemist, one character is seen cleaning (or polishing or something) an unused arm canon. And a much earlier episode featured a man with an "arm gun". Winry also mentioned that she learned how to make an auto-mail Machine gun, causing Ed to ask her if she's a Mad Scientist.
Panynia has a leg cannon.
On top of that, both Edward and Wrath (2003 anime version) have transmuted their arms to machine guns.
Anyone wondering where he gets the ammunition for it?
In the flashbacks to the Ishval campaign, Brigadier General Basque Gran uses a Philosopher's Stone and suddenly has cannons sticking out of his everywhere.
A one shot videogame/Filler villain used an automail arm with a double barrel gattling gun, he heavily implied that he had them cut off and replaced intentionally to be a more effective soldier.
A character in episode 3 of Samurai 7 had a drop-down gun in his arm that fires big needles from a revolver-like chamber. It was fired by flexing the wrist.
The Jeeg Bazooka was, in the anime Kotetsu Jeeg, a fairly large bazooka that replaced his arm. In Koutetsushin Jeeg, it becomes large enough for him to "hide" in — a BFG.
Allen Walker can turn his (non-artificial) left arm into a cannon that shoots a sort of paling projectile.
As mentioned in Artificial Limbs, one of Rally Vincent's recurring enemies in the manga series of Gunsmith Cats is a tall imposing man named Gray. He lost his hand in their first encounter and replaced his entire forearm with a multi-tooled Arm Cannon. Attachments included a hook and spring-launched machete.
The Type 99 Helldiver military labor from Patlabor mounts a 40mm autocannon on its right arm.
Guu from Haré+Guu can change one of her arms into a Gatling gun (complete with spent shells popping out of her mouth). Like the rest of Guu, it's inexplicable.
The gun-swordsmen in Brigadoon: Marin and Melan have a gun at the end of one arm and a sword at the end of the other, hence the name. They also have a second set of odd-looking retractable arms, which have stubby hands that they use for holding things.
In the art book Drawing and Painting the Undead, one creature, the Mechanized Corpse, had it's left arm replaced with an M60 machine gun. The ammo belt replaced its stomach and intestines.
Cyborg of Teen Titans can transform one arm (both in some stories) into a "sonic cannon".
In one of the many, many attempts by the DC writers to counteract the whole Superfriendscock-up, Aquaman had his arm chewed off by piranha when his powers were disabled. He got it replaced with a harpoon, and later a cybernetic hand that could morph into a harpoon gun. The hand was later replaced with one made of mystical water. Neither really helped much.
The KGBeast, a Russian Batman villain hacked off his arm and replaced it with a gun.
The Marvel Universe Villain Bushwacker can transform his right arm into a gun, thanks to CIA experiments preformed on him. He has to swallow bullets whole to provide ammunition.
The Amory Wars: Coheed has his right arm end up one of these. It's quite possibly the most realistic power of all the Kilgannons. (Coheed's left arm has several curved blades that pop out of it, as well as being able to destroy stars; his wife, Cambria, has tele-everything [-kinesis, telepathy, clairvoyance...], their surviving child, their son Claudio, has natural star-killing abilities, and can aparently melt through walls. As well as clairvoyance.)
Minor X-Men character Random had the power to turn his arms into protoplasm-firing guns.
Deadshot, Batman villain, sports two specially designed guns; one on each hand (the ammo belt goes around the wrist like a bracelet).
The BATs (Battle Android Troopers) of G.I. Joe had a variety of arm cannons including machine guns and flamethrowers. Depending on the incarnation involved, they either had to be manually swapped out for hands, or transformed automatically.
In the British comic Storm Force, John Storm had his left arm amputated above the elbow: he then carried around a rucksack full of prosthetic weapons, e.g. a machine gun arm and a flamethrower arm. He could also connect his stump to vehicles, e.g. a car or an aircraft, as long as they were designed for this.
Another Marvel Universe villain, Rapido, has a gatling gun mounted in place of his missing arm.
And Marvel anti-hero cyborg Death's Head II has a very high tech right hand (from the future!) that can transform into, among other things, a BFG.
In the 2000 AD series Sinister Dexter, the character Kal Cutter has a Mangapore skin-job replacing his right hand - when he points his hand like a gun and says "skin", his hand turns into a pistol. If he then says "hush" it grows a silencer.
The Transformers comics of Marvel, Dreamwave and IDW featured the chess master-type villain Shockwave, who not only sported a cannon as his left arm, but also transformed into a gun - making him a relatively rare example of Entire Body Cannon.
Megatron (The original, G1-version) himself might also qualify in that, while his right arm technically wasn't a cannon on its own, his famous Fusion Cannon was permanently attached to it, making his right hand almost useless to do anything (except perhaps point at things).
In the Marvel Star Wars comic book, there's Valance, a cyborg bounty hunter who predates Boba Fett. He had a blaster cannon fitted inside an artificial arm; it was powerful enough to take down a wall, and even come very close to killing Darth Vader himself.
Recently, her cybernetic parts were magically returned to organic by Ixis Naugus, which among other things means she no longer has one.
The last issue of The Amazing Adventures of the Escapist doesn't feature the Escapist at all, but rather his Empire Comics stablemate Mr. Machine Gun. Mr. Machine Gun has a cybernetic hand that can turn into a powerful submachine gun, magazine included — and he's a hero!
Dick Tracy villain Dr Plain had a small flamethrower built into his artificial arm.
Alpha from Clash of the Elements has the ability to create one of these from both of his robotic arms. The primary function of them seems to be to shoot ionic energy in the form of a concentrated laser beam, but only time will tell if they are capable of shooting more.
Films — Animation
Rattlesnake Jake from Rango has a gatling gun where his rattle should be. Instead of shaking his rattle as a warning, he spins the barrels. It's really creepy.
In Disney's Treasure Planet, Long John Silver was a cyborg and had an arm that transformed into a cannon at one point.
EVE from WALL•E. Like Mega Man, she can morph it from gun to hand, and even a little transitional mode which she uses most of the time. It's even detachable, and has an outside trigger which lets it be used by other characters as a conventional weapon. Darn it, WALL-E.
Films — Live-Action
In RoboCop 3, the eponymous hero got an upgrade: he can now detach one of his forearms and attach a combination machine gun, flamethrower, and rocket launcher when he knows he's facing a serious firefight. Or when he's just plain pissed off.
All the Transformers, really. With few exceptions, the Autobots and Decepticons convert their arms into guns, swords, and other weapons, in order to avert the use of Hammerspace. This is sometimes described as them "generating" the weapon out of the protoform.
The heroine of the Japanese film Kataude mashin g? (a.k.a. The Machine Girl) is tortured and has her arm cut off by the Yakuza. The owners of a machine shop who rescue her replace it with a machine gun.
David Weber's Honor Harrington had a pulser (hypervelocity needle launcher) built into her cybernetic arm as a holdout/emergency weapon. She needs a fingertip replaced whenever she uses it, but it does the job of a hand well enough and is almost impossible to spot.
The Cobra cyborg warriors in Timothy Zahn's The Cobra Trilogy. Each Cobra warrior (the good guys, by the way) has a laser built into each hand, a powerful shock launcher capable of frying electrical equipment, and a tank-killer class laser built into their leg. All without any external cues that would indicate their augmentation.
In The Oregon Files, Juan Cabrillo, the leader of a team of heroic mercenaries, has a high-caliber pistol built into a false foot — in addition to a number of other goodies, making it something of a Swiss Army Foot.
Aside from many obvious instances mounted on the Humongous Mecha of the setting, two separate characters (who never actually meet) in the BattleTech Warrior trilogy have custom laser weapons built into their prosthetic forearms. One of them finds his rather handy a couple of times when no other gun is available and just slugging an opponent with his metal fist isn't an option; the other never actually gets the chance to use his because when finally confronted with a would-be assassin, the latter reveals that he'd already infiltrated his victim's home days before and among other things made sure to drain the weapon's power cell.
The Star WarsExpanded Universe includes a type of four-armed nanny droid with blasters hidden beneath detachable hands on two of the arms. They're intended for the kidnapping-vulnerable children of politicians and such. Leia's custom-modified model for her son Anakin included this function on all four arms.
Babylon 5: The episode “Infection” has an alien organic weapon which takes over a human and gives their body armor and an Arm Cannon.
Battlestar Galactica (Reimagined): Cylon Centurions from the re-imagined show have hands that transform into guns. Additionally, said hands are apparently bladed, as seen in one particularly ugly moment during an early sequence of the second season.
Cleopatra 2525: The Betrayers are robots disguised a humans whose arms can turn into laser cannons. Their creators - giant machines called Baileys, can also deploy arm-like appendages that fire energy bolts.
Autons (of the shop window dummy variety) have guns built into their right hands, which drop open to fire.
Although they hardly use them, so do the alternate universe Cybermen. Their original universe counterparts just carried guns.
Rory in The Pandorica Opens and The Big Bang.
The Gunslinger in A Town Called Mercy.
Kamen Rider Den-O: Deneb gives this an odd twist: his individual fingers are guns. When he merges with his partner Yuuto to form Kamen Rider Zeronos Vega Form, his hands turn into shoulder-mounted machineguns.
Riderman was notable for being a Rider whose only technological augmentation was his right arm, and just for that, he still threw down more than any regular human ought to be able to. The arm itself could change somehow, through cassette cartridges inserted in his right elbow, into various forms (a drill, a claw, a net launcher, the original Hookshot...) but it wasn't until he showed up in Kamen Rider SPIRITS that he got a proper gun. And how! A proper Machinegun Arm that could, in a pinch, fire individual shells; seeing as how he just didn't have the firepower to take down some of his foes, these shells were hardening agents, on the theory that, if they couldn't attack him, he didn't actually need to blow them up.
The first Kamen Rider Decade movie gives Riderman (played here by Gackt) the Blaster Arm, a high-power energy cannon. However, unlike his others, he actually has to tear off his original cybernetic arm and attach the Blaster to the severed stump in order to use it.
Many Battlemechs in BattleTech mount a large gun where a hand would be, and leave the other arm free with a hand. Many other 'mechs do this with both arms, turning the 'mech into a dedicated firing platform at the cost of melee combat ability.
The Obliterator cult from Warhammer 40,000 is a cult of extremely chaos-mutated space marines whose bodies have been literally fused to their own weaponry and armour, allowing them to morph their warped body parts (with arms being preferred) into various types of BFGs at will, literally turning themselves into walking humanoid heavy weapon platforms. The question of where the ammo to these weapons are kept is, perhaps, better left unasked.
It's implied that Obliterators can access the Warp and use its energy to power their weapons. They also don't have kinetic-energy weapons anymore, so there's no point wondering where they store the ammo for heavy bolters or autocannons.
Inquisitor specified that they produce their own ammunition as a result of their encounter with The Virus.
Tau Battlesuits use this setup a fair bit as well, when they aren't shoulder-mounting it.
Combat servitors have had one or both of their arms replaced by weaponry of one sort or another.
Meanwhile, over in Warhammer, you've got the Skaven warlock Ikit Claw, so named because in place of one arm he's got a Magi Tech prosthesis. With a built-in warpfire thrower.
The Shadowrun RPG offers these as a type of cyberware. The end of the gun is in the palm, and locking one's hand back links it to the rest of the gun barrel. Rarely used as typically not worth the cost or essence, it does allow one to use their hand though. There was also a picture of a character with the more traditional gun as a hand in the older Critter's book.
Though there are no official "standard" magic items to do such, creating a component for a warforged (from Dungeons & Dragons' Eberron campaign setting) that did something similar would be a matter of upgrading the damage an armbow could cause. In 4E a crossbow can be embedded with the "Armbow" enchantment; the main perk is not having to worry about reloading.
In second edition this was removed from their core Charms and became an Ink Monkeys aftermarket upgrade; while it still took a Charm slot, that selfsame Charm could be expanded to produce a secondary "siege devastator" mode, a sword, or any other piece of kit you could reasonably fit within an Alchemical's body.
The post-Steampunk roleplaying game Etherscope includes rules for both cybernaughtic (i.e. robotic) arms and integrated weaponry, allowing a character at second level to integrate a pistol, or even a miniaturized grenade launcher into their arms. This troper is currently playing a character whose pair of robot arms contain said grenade launcher, a large autocannon, and a steam-powered chainsaw.
The collectible miniatures game Monsterpocalypse includes Rogzor of the Planet Eaters, who has two arm cannons.
GURPS: At least one version of the Ultra-Tech supplement lets a gun be implanted inside a character's hand or arm. Alternately the entire arm can just be replaced with a weapon mount.
MechWarrior, the tabletop roleplaying companion game to the BattleTech tactical game, allowed any character who needed an artificial limb to replace one that they'd lost to also install special equipment in said replacement. Naturally, concealed weapons were included, ranging from the somewhat feasible (such as a hold-out pistol hidden in the wrist) to the highly unlikely (like an entire laser rifle built into the forearm). The rules didn't explicitly deny the inclusion of any weapons beyond Game Master fiat, and if a GM wasn't paying attention or didn't care, this could lead to things such as concealed double-barrel shotguns, concealed Gauss pistols, or concealed anti-'Mech rocket launchers.
In Advent Children he upgraded to a cybernetic arm that can shift to a Gatling gun or a particle beam cannon.
Jack in Ar tonelico has his left arm (and most of his torso) converted into machinery, with his arm doubling as a gatling gun. Of course, he only uses it in his powered-up state, and normally just fires a pistol that he holds in his right hand.
Mega Man (Classic) in many incarnations. All versions justify this by having his arm morph from the cannon to a regular hand.
Zero has a transforming handheld saber, shield, pogo lance, hookshot, and tonfas, and a looted energy pistol. It doesn't stop him from using the Z-knuckle to graft enemy turrets and flamethrowers onto his arm, classic-style, in the fourth game.
And on the subject of Mega Man Zero, Fighting Fefnir. By extension, Model F users in ZX, which actually falls into the second category.
In the Mega Man X series, Zero's arm cannon is actually of the second type, emerging from his forearm. Odd that it switched over between series, but whatcha gonna do?
Because he lost his body at the Time Skip between the X and Zero series. The so-called Z-Buster is a pistol he grabbed off a dead Red Shirt to fight the Pantheons.
Wild Dog in the Time Crisis games. He replaces the arm he lost in the first game with a machine gun, he adds a flamethrower and a rocket launcher in the third game, and in fourth game he adds a grappling hook and a tractor beam.
In Persona 3 Aigis gets the deluxe version: Many of her equippable weapons (including her starting one) feature an Arm Cannon that allows her to fire bullets from her fingertips.
Forcystus in Tales of Symphonia has one of these; it's never explained how he got it, since he's a minor boss villain, but given that he wears an eyepatch as well, it's likely a replacement for a battle injury.
In Super Mario RPG, all weapons equippable by Geno have to do with some sort of gun. Standard guns shoot from his fingertip, his punches launch his forearms at the enemy, and one weapon, the "Hand Cannon", actually shoots canonballs from his elbow.
The towering Cyberdemon from Doom has a rocket launcher instead of his left forearm, and the grotesquely obese mancubus from Doom 2 has both of his hands replaced by rockets as well.
The Rifle Demons in the Disgaea series have arm cannons that fire energy blasts and can also generate a Tractor Beam and Laser Blade in some of the games.
In the Armored Core series, arms with integrated weapons are possible part selections for your Humongous Mecha. Practically every type of weapon is available — arm machine guns, arm beam swords, arm beam cannons, arm bazookas... Weapon arms are much more weight- and energy-efficient than hands holding guns, though less destructive and less versatile. This doesn't stop the EKLAKH machine gun arms from ripping whatever they target to pieces.
The Pokémon Rhyperior has holes in its palms that serve as cannons; it shoves rocks and the rare Geodude down the holes and shoots them. This is how it uses its signature move Rock Wrecker (Rock Cannon in Japanese).
At the end of Metal Gear Solid, Gray Fox shows up wielding a laser cannon arm; with absolutely no explanation as to where it came from.
The Russian cybernetic supersoldier Volkov from Command & Conquer: Red Alert has his arm turned into a tank cannon. Depending on the level, he may use a silenced pistol instead.
Also Cyborgs in Tiberian Sun and Kane's Wrath.
Yakuza Dead Souls has Ryuji Goda, whose right arm is replaced with a prosthetic that can transform into a gatling gun.
Reisen from the Touhou shooting game series is shown to fire her bullets straight out of her fingers (often with her hand in the classic invisible gun pose).
And Subterranean Animism gives us Utsuho Reiuji, whose right forearm is a nuclear control rod capable of firing small suns. Though it wasn't confirm as an arm cannon until Hisoutensoku, fandom nonetheless ran with it (skip to 2:00).
Fallout 3 has Protectrons whose arms have integrated laser cannons. And of course the badass Sentrybots whose one arm is a gattling laser cannon while the other is a rocket launcher. "Hostile detected. Non-combatant safety can no longer be guaranteed." No shit...
In Metal Arms: Glitch in the System, every robot with a dedicated weapon has them built in (save for Guards). Most Mil enemies even have dual arm cannons (and in one case, QUADRUPLE arm cannons), were Mil Grunts and Droid Miners only replace one arm. Somewhat Justified in that they're, y'know, robots that are actually made to fight, and it makes sense to integrate the weapons into their frames. However, in the case of the Droids, it's implied that they even have Arm Tools, and that they can actually swap various weapons and tools out as needed (This also makes sense; it has the weapon/tool's fire control and power supply directly connected to the Droid it's attached to, while still allowing for modular versatility). Glitch is the only one who demonstrates this, however. He even has a little hand he can snap onto his arm in place of a weapon, although it it functionally useless.
General Warfield from Starcraft II: Wings of Liberty gets one of these after losing his arm to hydralisk venom. It transforms from fully-articulated arm and hand to built-in gun and back.
In Mass Effect 3, Cannibals, batarian-derived Husks use arm cannons that are actually human bodies grafted onto their own. It's a combination of a machine gun and grenade launcher.
Scions are built around their massive arm cannon. In 2, it shoots an anti-infantry shockwave that deals heavy damage and knocks you out of cover. In 3, it's upgraded to a burst fire explosive energy cannon, that fires extremely accurate and powerful projectiles that do more damage than any other ranged attack in the entire game- including the anti-tank rockets Atlas Mechs shoot.
In The Reconstruction and I Miss the Sunrise, Tezkhra is in possession of one. It runs on energy collection technology he designed himself, and part of the reason for adding it instead of a regular cybernetic arm was field test data.
BioForge: One of the enhancements that the protagonist got as part of his Unwilling Roboticization. As the weapon is small and concealed, he doesn't even realize it for the longest time.
Brad Fang of Contra: Hard Corps has a gun grafted onto his arm which can function as a machine gun, vulcan cannon, or flamethrower.
Evolva: Switching into several of the weapons, like the Spore, makes the Genohunters' hands transform into this.
Ghost Song: A Journey Of Hope: In direct reference to Samus Aran, the protagonist's Deadsuit has one for a left hand. The fact that it cuts off at the wrist has disturbing implications for the person inside it.
Michelle Flammel from Monsterful, when she fuses with her golem Ourox she gains the "Oux Cannon" which is able to fire concentrated "moe energy", the cannon has been shown to be able to upgrade itself, 2 versions have been shown so far, 4k and 5k.
In Demon Fist, Jacob's Hook Hand shoots out on a chain, as a combination grappling hook and harpoon.
Two characters in Dino Attack RPG have an arm cannon. Dr. Cyborg's right arm becomes one when he switches into battle mode, and Sam Throramebi's prosthetic mechanical arm has a retractable hand that doubles as a plasma cannon.
Mecha Sonic of Super Mario Bros. Z has a machine gun/missile launcher hidden in both arms (though he only ever uses one at a time).
Loads and loads of Transformers, though the original Megatron is probably the most prominent example.
Honorable Mention goes to Beast Wars Megatron, who had twoludicrously big arm cannons in his original form. They became the head and tail of his T. rex mode. Thankfully he could remove one of them to use his hand.
A trait that carries over to most incarnations of Shockwave.
Animated Omega Supreme is another both-arms example. Of course, he was built to be the Autobots' war-ending Person of Mass Destruction, and is the size of a skyscraper in any case: he's not going to be able to physically interact with most creatures.
When Lightning Lad of the Legion of Super HeroesAnimated Series has an arm blown off by Big Bad Imperiex, it's replaced with a large mechanical replacement that can be used to channel and amplify his powers. He's surprisingly okay with the loss of his limb. It probably has more to do, though, with finally getting his sister back than with the cool replacement.
A video game section of Futurama parodies this trope by having the digital version character fire from their fingers, and in Fry's case, from his arm, which makes the sound of a shot gun cocking.
Zachary Foxx from Adventures of the Galaxy Rangers has a bionic arm. It needs a bit of charge time before firing, making it an arm-sized Wave Motion Gun more than an Arm Cannon. "Armada" showed it was capable of knocking a small starship off-course.
In the Animated Series of The Mask, the titular character has a pirate form that has an appropriately-styled cannon replacing one arm.
In Code Lyoko, amongst XANA's monsters the "Tarantula" has two laser cannons stubs in lieu of arms (and legs too, though the latter are never seen in use). Its firing position is a crouch reminiscent of Star Wars' Droidekas.
There is at least one real-life example of a machinist building such a device, so a one-armed man could compete in target shooting. In a strange case of life imitating fiction, he was eventually barred from competing with the "arm cannon" because it made him such a good shot — the gun was triggered by his opposite shoulder muscle and bolted on firmly, giving much more stability when firing than anyone could otherwise achieve.
Jay J. Armes, because of an accident involving railroad explosives at the age of 7 both his hands were replaced with prosthetics. At some point a gun was attached.
Dragon Ball Z: Frieza's lowly soldiers in the Planet Trade organization often use small arm cannons, though usually to no avail against far stronger foes, as these guys are nothing but cannon fodder for the main characters. Seems like the main use for these weapons is to conserve those fighters' meager store of power.
In Tsubasa Reservoir Chronicle, when the group first comes across a kudan gang war, the gang members are aiming their arms at each other while shooting off energy from just above their wrists, clearly meant to evoke the arm-mounted cannon.
The Gundam "Alex" in Mobile Suit Gundam 0080: War in the Pocket has a Gatling gun hidden in each forearm, which came as a nasty surprise to the Zeon commando who tried to take it out.
Likewise, the Shining Gundam has a pair of light beam guns on its arms.
And before either of those, the Zeta Gundam had twin grenade launchers.
In Gundam 00, all of the Human Reform League's mobile suits either had an arm-mounted cannon, or a BFG and sensor array replacing the head
And don't forget Gundam Wing's pre-redesign Gundam Heavyarms, which sported a massive Gatling gun on it's left arm... and two more in the chest-plates... not to mention the multiple rocket launchers everywhere they could fit them. More Dakka AND Macross Missile Massacre, anyone?
Post-redesign in Endless waltz switched the one gun on the left arm to -two- massive gatling guns on the right arm, and still keeping the rest of its armament.
James Bond gets one in Moonraker. It's easily concealed and fires darts tipped with either poison or explosives.
The 1982 film The Soldier features a scene in which a character has a sawed-off shotgun strapped to his arm and hidden by his sleeve. When stopped by a policeman or security guard and asked to put his hands up, he complies... and uses the gun to shoot the guard in the chest.
The aliens in Battle: Los Angeles have guns surgically attached to their arms, prompting the Marines to remark that they are committed to their cause.
In Cowboys and Aliens, The main character steals one in his escape from the Aliens, and the only thing that has a chance of damaging the Aliens.
The Pyrrans in Harry Harrison's Deathworld are born into the losing side of a technology-vs-biology conflict that has an entire planet forcibly mutating to attack them. They train, from about the time they're old enough to walk, with forearm-mounted holsters which automatically slap a pistol into the wearer's hand when he or she positions the fingers for gripping the pistol's butt. If the fingers aren't crooked just right, the trigger finger may touch the trigger before the firer intends, which will cause problems if he or she didn't mean to shoot just yet. These are large-caliber pistols using live explosive rounds, making them exemplars not only of arm-cannons, but several other kinds of insanity as well.
One of the first appearances of cybermancy in Shadowrun was when three "Hellions" - heavily modified cyborgs - appeared in one of the novels. Each had a triple-barreled machine gun incorporated into one arm.
The Hellions appeared in "Never Trust an Elf".
One of the weapons used by the titular Fighters of Danveyt, a novel by Mikhail Akhmanov, is a fingerless glove that has various weapons "attached" to each finger. One of the most prominent weapons is the freezer, which creates a short-lived miniature black hole at the designated point, causing the temperature in the immediate vicinity to be dropped nearly instantly to extremely low levels (hence the name). For bonus points, there is also a rapid-fire version of the weapon.
A character in the book had previously lost an arm in a war, so he replaced it with a prosthetic that has these weapons built into it, making this an example of the first case.
One of the weapons used in the Death Zone novels set in the S.T.A.L.K.E.R. universe is the armgun - a powerful laser emitter mounted on the forearm. At full power, it can slice a staltech in half.
In Steve Perry's Matador Series, the signature weapon of the Matadors (and Emile Khadaji, who inspired them) is the spetsdöd. It's a dart gun mounted on the back of the hand, and fired by extending your index finger and touching the barrel. All you have to do to shoot something is point at it. The darts are tiny, and thus technically nonlethal, but they can be loaded with nearly any sort of drug or poison.
Spectral Shadows has Princess Electra in Serial 2, who pretty much is a Mega Man type character, if only an animal character instead of a human.
The Kull Warriors from Stargate SG-1 have an energy weapon mounted on both arms, which is essentially a miniature, rapid-fire staff weapon.
Soaron and Blastarr from have arm-mounted Digitizers attached to their forearms; they slide over their fist and lock into position when used to digitize a hapless human victim. Blastarr also has cannons on his fingertips.
Additionally, Hawk from the Future Force has a miniature rocket launcher mounted on top of his arm. It fires tiny but devastating missiles.
The Interceptor in Interceptor had his zapper mounted to his left sleeve.
One of the perks of being a Protector in Earth: Final Conflict is getting your very own organic arm cannon.
The Tavleks mercenaries in Farscape are armed with a gauntlet that can fire energy blasts, create force-fields, and inject the wielder with a powerful stimulant. Unfortunately, though the stimulant enhances strength and speed, it also has a nasty habit of giving new users delusions of grandeur. The good news is that knocking the wearer out automatically detatches the gauntlet, thus making sure that they can be disarmed without being dis-armed.
Mighty Morphin' Power Rangers: Rito Revolto had a flamethrower that mounted like a glove over his left hand. It was only used in his battles against the Ninja Megazord (the second of which was a recyc of part of the first).
HITMarks from Mage: The Ascension, while more commonly shown using the gatling guns stored in their backs, do have machine guns in their arms as well.
Both the first and second kind of Arm Cannon are available in Rifts. The rulebook specifically points out the difficulties of the Type One Arm Cannon, but justifies it by pointing out that hands are tricky to get right, and Rifts is set in a (borderline) Scavenger World.
In Warhammer 40,000, all terminators have the options of taking arm-mounted cannons of one kind or another.
Some BattleTechBattleMechs do this as well, mounting one or more weapons atop or on the outer edge of the forearm. Early examples from the franchise include the Shadow Hawk and Thunderbolt, which feature some of the largest gunbarrels relative to their size, while the contemporaneous Phoenix Hawk featured smaller weapons in double mounts on the forearms to complement its large laser rifle. Perhaps most impressive is the Thunder Hawk, featuring cannon barrels so grotesquely massive that they actually constitute a larger percentage of the left arm than the rest of the arm itself.
The Transhuman Space setting features a realistic version in the form of "weapon pods"; forearm-mounted modules, usually with two different guns integrated into them. (In fact, these appear in the game because one or two discussions of possible future infantry equipment at the time of its publication actually included the idea.)
All of the MAX Suits in PlanetSide 1 utilize this. The Terran Republic has a gun fitted onto each arm behind the gloves, whereas the New Conglomerate and Vanu Sovereignty has a single big gun on the right arm instead of hands. Planetside 2 has the New Conglomerate using this as well for their (now akimbo) cannons, while the Vanu Sovereignty MAX shifts to carrying a large cannon in each hand.
The Majestic XII Commandos from Deus Ex have some sort of devices built into the arms of their suits that packs the capabilities of pretty much all the normal weapons in the game (assault rifle, sniper rifle, flamethrower, rocket launcher). To fire, they level their arms at their target while clenching their fists.
Not quite a gun, but JC's minicrossbow is attached to his arm.
The Alt Eisen and Weiss Ritter in Super Robot Wars both have guns mounted on their left arms.
This comes from being based on the Gespenst, since every type of Gespenst has three barrels mounted on their right arm. Strangle only the Wei?Ritter and Alt Eisen actualy have build in weapons which use them as range attack and the Gespenst II M, makes use of the barrels as close range weapon. Which makes someone wonder why the other types have those barrels but no weapon build in to use them.
On the older Gespensts (and the MK II M), those are apparently the hilts of their Beam Sabers. The MK II M's are designed to be able to do the Jet Magnum attack.
In Front Mission (SNES) and its later PlayStation remake (and the Nintendo DS port of that) named Front Mission 1st, at least one of the early game arm parts has an integral machine gun; it's strong, has good accuracy for the early game and can save you money due to not having to buy a separate machine gun for a regular arm. Its only downsides are the inability to unequip, the weight, and those of a machine gun (as a Short range weapon its priority is higher than Melee weapons but lower than Long range weapons, and between Short range weapons its priority is less than a Rifle which only shoots once per attack).
Slipskulls in Resistance: Fall of Man have their guns mounted to their arms with metal bands. When you find a pair of their weapons lying around in the New Game + , you just use them like regular machine pistols, though.
As of Touhou 12.3 Hisoutensoku Utsuho's control rod canonically functions as one of these.
In World of Warcraft, Grandmaster Engineers can attach hand-mounted rocket launchers to their gloves.
Colonel Daren from Red Dead Revolver has an arm cannon after certain events in the beginning of the game.
Rinoa Heartilly from Final Fantasy VIII has a weapon called "Blaster Edge" on her arm, which can launch a variety of disc-like projectiles in a boomerang trajectory. In her Limit Break "Angelo Cannon," she can shoot her dog like this. (No, not that way. This way.)
In Assassin's Creed II, Leonardo da Vinci builds a pistol modification into Ezio's hidden blade gauntlet quite late into the game. Maybe you would expect that it allows for some stealth, until the sound it makes almost makes you think that they're making a fireworks missile explode right next to your ear. It does One-Hit Kill everyone, though, even the Brutes.
As a fair warning to other tropers, The POPE, and Cesare are both Immune to Bullets. This is strange since the first is an old man in robes, and the other is a whiny 20 year old in light armor (stops "two-handed" claymore swords too...)
Which is something of a moment of fridge brilliance, come to think of it. One would suppose the Animus disregards the bullets because Ezio doesn't remember either of them dying that way. He saw Rodrigo die by a poison apple, and dropped Cesare off a high wall.
Subject Delta from BioShock 2 has all his guns bolted to a plate on the back of his right hand.
Firebats from StarCraft have twin, wrist-mounted flamethrowers. This continues on in the Starcraft II unit, the Marauder, who has twin grenade launchers for arms. It's a giant Mini-Mecha / Powered Armor suit, though, so he can still take it off. The new Firebat has the same design as well.
Shermans from Fracture have had one of their arms replaced by a bio-engineered grenade launcher.
In the Daud DLC of Dishonored, Daud has a wrist-mounted crossbow as one of his ranged weapons. As well as functioning like the crossbow in the main game, it's capable of extending the bolt to pierce necks for stealth kills, similar to the Hidden Blade of Assassin's Creed.
WildStar's Warriors have a Power Sword and the "XL45 Hell Mitten," an arm-mounted device that acts as a cannon, a shield, a chainsaw, and many other deadly and practical devices.
Animated Swindle actually has a BFG that he attaches to his wrist like Megatron, and a pair of retractable cannons on each arm. This is in addition to his over-the-shoulder lasers, and the gatling gun coming out of his chest.
Every robot or cyborg character on Gargoyles has a retractable particle beam gun in its forearm, with an almost identical extend/retract mechanism. Sensible, since all of them were evolved from designs stolen from Cyberbiotics by Xanatos, and it's hard to reinvent the wheel when you didn't invent it in the first place.
Buzz Lightyear, of Toy Story and Buzz Lightyear of Star Command fame, has a laser built into the wrist of his suit — at least on the latter where it's standard-issue; on the former it's "a little lightbulb that blinks".
His team in the latter had them as well, except for XR, had neither this, nor the other version, but had a much larger arsenal, part and parcel with being a robot. The wrist-mounted laser seems to be part and parcel of the standard issue Star Command combat suit.
Warp Darkmatter is somewhere between this and and his arm being the cannon. Normally, he has a robotic claw hand at the end of his artificial right arm, though it has been switched out with a laser cannon and a flamethrower.
Odd from Code Lyoko, in his Lyoko Avatar, fires "Lazer Arrows" from his wrists.