Backpack Cannon

The Ammunition Backpack is cool and all, but having to lug both it and your gun may be a hassle. Why not combine them both and get yourself a Backpack Cannon! Yeah, it would probably be really hard to aim, but come on.

Compare Shoulder Cannon, Weaponized Animal, Jet Pack, and Weaponized Exhaust.

Anime and Manga

 * General Catton in Samurai Pizza Cats had a double backpack cannon.
 * Throughout the Gundam franchise, there have been several examples.
 * The GM Cannon 2 from Mobile Suit Gundam 0083 Stardust Memory features a double backpack cannon. Its predecessors, the Guncannon and original GM Cannon, had shoulder-mounted cannons.
 * From the same series, there's the Xamel and it's folding BFG. It's a bit hard to tell if it is this or a Shoulder Cannon since it's not entirely clear given the Xamel's unusual shape, but as far as anyone can see it's actual shoulders are bare.
 * The Zaku Cannon, introduced in Mobile Suit Variations, was found to be unstable due to its backpack being too heavy and the recoil being too great, though some units were later used by the Earth Federation in Mobile Suit Zeta Gundam, and by Zeon remnants years later in Gundam Unicorn.
 * The RX-78-01 from Gundam: The Origin had a backpack-mounted cannon and one beam sword rack, instead of the traditional pair of saber racks. The RX-78-02, Amuro's Gundam, could have one of the racks replaced by a similar cannon.
 * The OZ-04MS Tragos from Gundam Wing featured a backpack cannon and a hoverskirt that made it resemble the original Guntank, although both could be detached to allow the Tragos to function as a conventional mobile suit as well.
 * The titular Gundam X features the backpack-mounted Satellite Cannon system. Its Mid Season Upgrade, the DoubleX, naturally has twin cannons.
 * The Lancelot Conquista's main weapon is the large Hadron Blaster on it's back that folds forward and docks with the VARIS rifle in order to fire. And it's so freaking powerful that nothing except the Guren SEITEN's upgraded radiant wave surger can stop it.

Comic Books

 * In Transformers Last Stand of the Wreckers, Ironfist's lightformer cannon is his backpack... but he has to take it off to use it.
 * The tank mode turret of Ironfist's teammate Guzzle is situated on his back when in robot mode, with the barrel extending far past his head.

Film - Live Action

 * From Star Wars, Boba and Jango Fett's Jetpacks can fire rockets.

Tabletop Games

 * Shadowrun. The Ballista Multi-Role Missile Launcher, which is worn as a backpack.
 * Battlelords of the 23rd Century. The Cizerack (large quadrupedal catlike aliens) have been known to wear weapon systems mounted on their backs. See a picture here.
 * Likewise Battle Cattle (pic here).
 * Most of the power armor in Rifts has this, notably the Glitterboy and its variant armor.
 * In Warhammer 40000, the Tyranid Biovore is a living Backpack Cannon.

Video Games

 * Mario's F.L.U.D.D. in Super Mario Sunshine
 * The Buster Pod from Custom Robo Arena, which was just a cannon mounted on your robo's back.
 * Pokémon Black and White: Genesect, a Pokemon with a mounted cannon on its back, as modified by Team Plasma.
 * Blastoise is a giant turtle with two cannons in its shell!
 * Carol from Wild Arms 5 uses a backpack-mounted missile launcher. She can't use it without falling on her ass, but it does pack quite a punch.
 * The Taraba ninja from Shinobido use these as weapons. Yup, cannons, right...
 * Shadow from Mega Man X Command Mission has a "Pulverizer Cannon" on his back. It takes three turns to charge, and like its name suggests, it hurts a lot on all your character.
 * The Dwarf from Sacred has this as a special weapon.
 * The Odin superheavy siege walker from Starcraft II features a set of four backpack-style guns, described as 330mm high-ex cannon (you could fire a smallish person out of a gun like that). The Odin actually has to hunch back and take up a firing stance before using the things, but it's mighty impressive when it does.
 * GameMods for Starsiege Tribes and Tribes 2 usually add at least one of these. Instead of carrying around a regular backpack, like a shield pack, you can stick a big gun on your shoulder, allowing you to use both your hand weapon and the backpack weapon at the same time.