There aren't many muzzle brakes for shotguns being made for a couple of reasons.
*They are very loud.
*A shotgun doesn't produce enough pressure at the muzzle for a brake to have much effect.
*There are potential problems with wads catching in the baffles and spitting shot out sideways to the direction of fire.
*Did I mention they are very loud?
Mostly, you see barrel porting being used on shotguns, which consist of a series of small holes drilled in the barrel. This addresses the snagged wad and spitting shot problem.
But they are very loud!
rc