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.
Muzzle brakes are simple in concept. One of the simplest designs can be found on U.S. 90 mm tank guns, which consisted of a small length of tubing mounted at right angles to the end of the barrel. As the shell left the barrel the gasses would rush around it, hitting the inside "front" of this tube, pushing it, and thus the gun as a whole, forward.
Brakes more typically consist of a small length of tubing "fit over" the end of the barrel, pointing in the same direction. 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 this style of 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 turbulence from the escaping gasses affects the trajectory.
Muzzle brakes on both small arms and artillery pieces can also cause escaping gasses to throw up dust clouds, obscuring visibility and making the firer easier to spot by the enemy. Troops often wet the ground in front of antitank guns to prevent this. At night, muzzle brakes can increase a weapon's muzzle flash.