Muzzle brake
A muzzle brake is a device that is affixed to the muzzle of a firearm, and which redirects propellant gases to either counter the recoil of the gun, or to prevent the muzzle from climbing during rapid fire. They are very useful for combat and timed competition shooting, and are commonly found on rifles firing very large cartridges (often big-game rifles), as well as some artillery and tank guns.
Brakes typically consist of a small length of tubing, slightly larger than the barrel. Slits or holes are cut into the tube, angled toward the rear of the gun. When a round is fired there is a brief period in which the bullet is at the end of the brake, but not yet exited. During this time the gasses pushing the round escape through the holes to the rear, countering the recoil.
The major disadvantage to using a muzzle brake is the fact that they necessarily increase the noise level of the report dramatically; generally, the more effective the muzzle brake is, the louder it will be. Brakes often also slightly decrease the accuracy of the round, as turbulance from the escaping gasses effects the trajectory.