Barrel ports or muzzle brakes redirect the power gasses in a direction that does not add to or even cancels some of the recoil.
These powder gasses weigh just as much as the original powder charge that generated them.
With a target load, you have about 18-20 grains of powder and 580 grains of shot. The mass of the shot is responsible for almost all of the recoil so taking that 18-20 grains of powder out of the equation will barely be felt.
With rifle loads, you might have 45-50 grains of powder accelerating a 150 grain bullet. Removing the powder's contribution to recoil will be noticable.
As you get to the 4000+ fps varmint rifles, the powder often outweighs the bullet. A 100% effficient muzzle brake could theoretically make such a gun kick forward.
A muzzle brake might be effective in a shotgun for black powder loads, where the powder charge weighs 80-90 grains.