[Copland]

Below is info. from a couple of sites:

Newton's law says (to paraphrase) that for every action there must be an equal and opposite reaction. Recoil (kick) is the rifle's reaction to the action of the bullet being accelerated down the rifle barrel. Recoil is caused by two factors. The first is the bullet itself, which cannot be interfered with. The second is the expanding powder gasses that are pushing the bullet and must also exit the barrel. Muzzle brakes reduce recoil by diverting part of these gasses to the side so that they do not add to the rearward recoil.

A muzzle brake or recoil compensator is a device that is fitted to the muzzle of a firearm or cannon to redirect propellant gases with the effect of countering both recoil of the gun and unwanted rising of the barrel during rapid fire.