Arm Cannon

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. Or stick a hardpoint on that limb (if you cared about subtlety, you won't consider Arm Cannon) to swap weapons as needed, and use said limb normally while armed. A specialized 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 weaponized vambrace. Occasionally, the weapon may fit over the forearm and hand, giving the appearance of the first kind. 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. 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.

Examples of Arm Cannon include:

First Kind (arm is cannon)

Anime and Manga

Dragonball 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.
- Also, 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.
- 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).
An early City Hunter villain had a machine gun replacing his right arm. After being defeated and mocked by Ryo, he took this trope up to eleven, and grafted a cannon in his leg.
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.
Guts in Berserk has a Steampunk mechanical 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.
Franky from One Piece has a machinegun/cannon infinite-ammo combination in his left hand. His right hand extends on a chain and is very strong. Both hands are also impervious to normal damage... although those are not the only weapons he has integrated into his body.
- Early villain Wapol can morph his arms and tongue into cannons.
Miyu in My-HiME and Mai-Otome can turn her arm into an energy gun and a flamethrower, in addition to having knee cannons.
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 (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.
After a major upgrade, Chachamaru of Mahou Sensei Negima now sports one of these.
- Sayo also gained one after Haruna made her a robot body.
The cyborg Talos from Crusher Joe has a prosthetic arm with a built-in machine gun and a prosthetic hand that fits on the end, concealing the gun from enemies.
In Yu Yu Hakusho, when Sensui switches to his Kazuya personality, his left hand morphs into a gun.
The Mariage from StrikerS Sound Stage X of the Magical Girl Lyrical Nanoha franchise. One of the many forms their arms can take is a high-explosive howitzer-style cannon which one of them used on an unprepared mage. Since the mage was Subaru, It doesn't work.
'D.Gray-man: 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.
In Bleach, Soi Fon's zanpakutou brings her under this trope for short periods of time. In shikai, her zanpakutou integrates with a finger on her right hand to turn her into the ultimate two-hit kill warrior. However, in bankai, her zanpakutou becomes a stinger missile launcher that is integrated with her entire right arm to turn her into the ultimate one-hit kill warrior.
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.
Voltes V's 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.
Nichijou: Nano has a machine gun in her right arm, which shoots beans. This is only Played for Laughs, though, and never used seriously.
Nurarihyon no Mago: Yura Keikain's shikigami Rentei transforms into one in Yura's arm that shoots a powerful stream of water.

Art

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.

Comic Books

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 Superfriends cock-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.
- Oh really?
- The animated version actually cut his own arm off when chained up, in order to save his infant son. He gets the harpoon replacement and sticks with it throughout the series. Can we say Badass?
In another DC example, Guy Gardner (one of the many Green Lanterns) turned out to have an alien lineage that gave him the ability to shift his body into any weapon. It was the Nineties, what can we say.
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 was a shapeshifter who frequently turned his arms into cannons made of flesh. It was never made clear what they are supposedly shooting.
- Projectiles made of protoplasm.
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.
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.
Deadshot, Batman villain, sports two specially designed guns; one on each hand (the ammo belt goes around the wrist like a bracelet).
Bunnie Rabbot from Archie's Sonic The Hedgehog sports one. However, since she is only partly cyborg, the power is drained straight from her life force.
- Recently^[when?], her cybernetic parts were magically returned to organic by Ixis Naugus, which among other things means she no longer has one.

Films -- Animation

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.
In Disney's Treasure Planet, Long John Silver was a cyborg and had an arm that transformed into a cannon at one point.
Emperor Zurg in Toy Story 2.
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.

Films -- Live Action

In RoboCop 3, the titular hero has gotten 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.
The T-X in Terminator 3 can turn its arm into an energy gun and a flamethrower, among other weapons.
Videodrome
In the 2007 Transformers film, Megatron turns his arms into a huge cannon!
- Megatron in Revenge of the Fallen has had his cannon integrated into one arm this time, also including a vicious blade weapon.
- 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.
Played for Laughs in the Grindhouse film trailer (and later real movie) Planet Terror where a one-legged stripper has a shotgun attached as her other leg. She is amazingly skilled with it.

Literature

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 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 Wars Expanded 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.
Justin Xiang Allard is featured to have one in the BattleTech book series. He lost his arm, and his cybernetic prosthesis has a laser weapon hidden inside.
Max Barry's Machine Man equips Dr. Charles Neumann with one after he's become a Man in the Machine. It's intended to be the first of an entire line of weaponry for Super Soldiers.

Live Action TV

Autons (of the shop window dummy variety in Doctor Who) 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.
Cylon Centurions from the re-imagined Battlestar Galactica 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.
In Blake's 7, Travis has an artificial left hand with a gun built into it.
Riderman, from Kamen Rider V 3, was notable for being a Kamen 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.
Deneb from Kamen Rider Den-O 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.
Adam, in season 4 of Buffy the Vampire Slayer, has both a mini-gun and a grenade launcher built into his arm.
The Betrayers in Cleopatra 2525 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.
The Babylon 5 episode “Infection” has an alien organic weapon which takes over a human and gives their body armor and an Arm Cannon.

Music

Video Game music band Arm Cannon warrants a mention. Fittingly, the first track of their first album is from Metroid, and the second is from Mega Man 3.

Tabletop Games

In Mortasheen, the creature Smoldron has a flamethrower version of this.
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.
Warhammer 40,000
- The Obliterator 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 Big Freaking Guns 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.
- Blood Angels have drum-fed Angelus bolter variant purpose made for wrist mounting, as standard issue for Sanguinary Guard (close combat oriented elite force).
- Space Marine Terminators have Assault Cannons, gatling type guns that are attached to one hand. Their Chaos counterparts have a similar gun, the Reaper Autocannon.
- Space Marines of the Grey Knights Chapter have wrist-mounted Storm Bolters, essentially two miniature rocket launchers firing as one weapon. This is generally fired by thought alone.
- Tau Battlesuits use this setup a fair bit as well, when they aren't shoulder-mounting it.
- Tyranid weapons are all organic parts of the wielder's body, but luckily they have backup arms...
- Combat servitors have had one or both of their arms replaced by weaponry of one sort or another.
Meanwhile, over in Warhammer Fantasy Battle, you've got the Skaven warlock Ikit Claw, so named because in place of one arm he's got a Magitech 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.
Alchemical Exalted can get a Charm installed that allows them to turn their arm into a gun that fires pure Essence, as demonstrated here by Elegant Nova of Progression.
- 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: Ultra-Tech lets a gun be hidden inside the 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.

Video Games

Barret in Final Fantasy VII switched his prosthetic arm to a gun when he got permanently pissed off, and changes back in supplementary materials once he's done.
- In Advent Children he upgraded to a cybernetic arm that can shift to a Gatling gun or a particle beam cannon.
- An interesting variation as in Advent Children it seems to need reloading once in a while.
  - It might not have been animated, but given the battle system used in Final Fantasy, he had plenty of time to reload after his turn.
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 in many incarnations. ~~Some~~ ~~Most~~ 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.
  - Model FX users just have Hand Cannons Akimbo.
- 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. This Troper has seen every type of weapon integrated into the arms—arm machine guns, arm beam swords, arm beam cannons, arm bazookas...
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).
- Magmortar does this too. You can see it at the beginning of Giratina and the Sky Warrior.
All Weapons wielded By heavy armour in Dystopia.
In Tribes: Vengeance. All Weapons For Heavy Armour are in the Arm cannon Variant
KOS-MOS from the Xenosaga series. It's usually just a regular arm, but she morphs it into an arm cannon for her R-Cannon attack.
Hunters from Halo have a fuel rod cannon (Covenant Rocket Launcher) attached in place of a right arm. At least they seem to use the left one to fire
Several bosses from Wolfenstein 3D have arm cannons, such as the Grosse Family and Adolf Hitler.
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.
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).
In Naruto Clash of Ninja Revolution 2 (and other releases, most likely), the game-exclusive character Bando can detach one of his hands and attach a Big Effin' Arm Cannon.
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...
Grobyc (note the Sdrawkcab significance) from Chrono Cross.
In Metal Arms, 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 Septerra Core, the bounty hunter Ayram had a pair of arm missiles. They even return after he fires them. He'd still rather have actual hands, though.

Ayram: "If I had hands I could create as you do."

The Barrelchest Mk II in RuneScape's "A Clockwork Syringe" quest is equipped with a full-size gunpowder cannon in place of its left hand.
Sonic Adventure: E-102 Gamma has one. His "brothers" Beta, Delta, and Epsilon each have two.
John Cain, the main character of the obscure PS 1 game One, had his left arm forcefully replaced with one of these.
Evil Dead: In the video game Evil Dead: A Fistful of Boomstick, Ash Williams, as an extension of his usage of right-hand replacement weapons, can swap out his chainsaw hand for a gatling gun.
In Mass Effect 3, "Cannibals", Batarian-derived Husks use arm cannons that are actually human bodies grafted onto their own.
Saints Row the Third the protagonist get's to use an Arm Cannon in the mission "https://deckers.die" and get to keep it after.
Many Strogg enemies in the Quake series, one even has a driller-arm which can shoot lightning out of it.
The King of Fighters: K9999 possesses an arm cannon as part of his transformation weapons

Web Comics

Reona from the web comic Zap lost her arm in the backstory, and had it replaced with a retractable arm cannon.
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.
The superhero Cannon from Heroes of C.R.A.S.H. has one as his superpower.
The Nemesite prison guard-bots in The Inexplicable Adventures of Bob have gun arms. When Fructose Riboflavin defeats them in his jailbreak, he removes their arms and mounts them on stands to use as Big Freaking Guns.
All characters in Bob and George based on the Mega Man sprite have the ability to morph their arm into a cannon. Except George.
Clayton from Chaos Theory has a frigging minigun built into his robot arm after a weapons malfunction caused him to lose the original left one.
Arlecchino from Commedia 2X00 has one. So far, he's only used it to blow up cantaloupes while bored, but it makes a PCHAO!! sound when he does it.
Girl Genius had "Mr. Tock" and Beetle's guard clanks, the mecha-chess combat vehicles used by the Knights of Jove, those zombies, and the old monks' clank with pincer/spray hybrid hands (spoiler). Wulfenbach Sentinels have both guns and tar throwers.

Web Original

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).
The titular protagonist of Quirky Misadventures of Soldine the Cyborg is equipped with a Black Box rocket launcher in the right hand and a large six-barrelled gatling gun in his left hand. His Evil Knockoff, Robosol, sports twin submachine guns.
The humanoid guard robots protecting the train in the RWBY "Black" Trailer have hands that can morph between miniguns and blades.

Western Animation

Loads and loads of Transformers.
- Honorable mention goes to Beast Wars Megatron, who had two ludicrously 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.
- Shockwave in Transformers Generation 1 has a blaster arm.
- 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.
- In Transformers Prime, almost every Autobot has one or two of these, the only exception being Ratchet, who sticks to arm-mounted blades.
When Lightning Lad of the Legion of Super Heroes Animated 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, though, and is more elated with the idea of finally getting his sister back.
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.
Can't forget Deadshot from Batman Gotham Knight... complete with dakka!
In the Teen Titans animated series, Cyborg has weaponry in almost all of his appendages. No, he's not kidding about the shoe rocket. His sonic cannons remain his favorite weapon, though.

Cyborg (filking the theme song): If there's trouble you know what to do! Call Cyborg! He can shoot a rocket from his shoe!

- Cyborg also sports two in DC Super Friends.
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.
The He Man and The Masters of The Universe villain Hordak, also the Big Bad from She-Ra: Princess of Power, could transform at will his arm into a cannon - a prominent use of his Voluntary Shapeshifting skillset; he could also turn his whole body into a rocket. The 1980s comics had the exact style of this vary wildly Depending on the Writer - sometimes the cannon was wider, sometimes narrower; generally it was his whole arm to the shoulder that transformed, while with certain illustrators it was just his forearm. Whatever form it took, it always looked awesomely Badass - Skeletor never pulled off any trick like that...

Real Life

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. He even got his own action figure.
A biological example is the pistol shrimp.

Second Kind (arm-mounted cannon)