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1.
Noun:

Megaman's preferred weapon; a retractable prosthesis that fires plasma charges.
"...and so, with a final blast of his arm cannon, Megaman laid the diabolical Dr. Wily's latest mad creation to waste!"
by Joshua B. Wright April 07, 2004
 
2.
A video camera, usually a larger videocassette-carrying type.
Even though you don't use an arm cannon anymore, the copyright mafia can still confiscate your Coolpix camera even if you don't take any videos with it.
by pentozali May 01, 2009
 
3.
A weapon that is worn over the arm instead of being held with a grip. It usually consists of a large metal tube that fits up to the user's elbow. The actual barrel lies in front of the user's hand, with an internal lever or trigger for firing the weapon. Outwards looks and internal workings differ, but since arm cannons are largely a work of fiction they usually fire some sort of energy projectile and draw their power from the environment or a power generator carried by the user.

Arm cannons are the primary weapon of video game heroes like Megaman and Samus Aran. Samus Aran's arm cannon is part of her hi-tech armor and draws it power both from it and from the environment. Megaman is a robot, and thus his weapon is actually built into the arm. When not in use, the cannon can be reconfigured to work like a regular hand.

In both cases, however, the arm cannon can be upgraded to fire lots of different projectiles without actually exchanging the cannon itself for a different weapon. Several beam weapons can also be charged before firing.

The advantage of an arm cannon is - in addition to the aforementioned lack of ammunition requirements - increased stability when firing the weapon, as the weapon's weight is balanced across the entire lower arm of the user. The recoil will also only push the user's arm backwards, rather than forcing the barrel into an upwards motion as with a pistol. This makes it much easier to keep the weapon stable when firing rapidly.
Samus pointed her arm cannon at her distant foe and pulled the trigger. An instant later, a red-hot plasma beam streaked towards its unfortunate target.
by bmthe12345 April 06, 2012