Mega Buster

The Mega Buster, also known as the Rock Buster, is the primary weapon wielded by the titular character of the Mega Man games. It is a small blue cannon that replaces Mega Man's right forearm and fires pellets of energy.

Features
The Mega Buster's ammunition is solar powered, allowing Mega Man to fire an unlimited amount of compressed energy bullets that could split boulders in half, usually in a three round burst. It has the ability to be charged up in later games, the longer the shot is charged the more destructive power the cannon gains.

Trivia

 * The Mega Buster is extremely similar to the Arm Cannon from the Metroid franchise. Both are right-forearm replacing cannons that can be charged up and utilized by an action game's main protagonist.