Muzzle brake
Muzzle brakes are simple in concept. One of the simplest designs can be found on US 90mm 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.
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