I will go through an example of a brake.
10 pound rifle, 100 gr powder 200 gr bullet at 3000 fps
The forward momentum of the gas center of mass is [100 gr][4,700 fps]
The forward momentum of the projectile is [200 gr] [3000 fps]
The rearward momentum of the rifle will be equal to the sum of the forward momentum of projectile plus gas. = 1,070,000 gr fps
The rearward velocity of the free recoiling gun will be its momentum divided by its weight.
1,070,000 gr fps /[10 pounds ][7000 gr/pound]]= 15.3 fps
If we had a gas brake that sent ALL the gas sideways the rifle recoil velocity in this case would be reduced to 8.6 fps
If we had a gas brake that sent ALL the gas rearward, the rifle recoil velocity in this case would be reduced to 1.8 fps.
We are still searching for gas brakes that work that well.