Shotgunjones hit the nail square on the head. The total recoil is based on the weight of the total ejecta, which includes the powder gases & the velocity it is ejected. The percentage of the powder charge in proportion to the projectile is much lower in a shotgun than most centerfire rifles, plus the rifle will normally have a higher muzzle pressure than the shotgun.
Porting works by diverting gasses which would normally eject forward being diverted outward, thus not adding to the recoil.
In a shotgun though the portion of the recoilbrought on by the powder gasses is much lower than in the rifle, so not as much to work with there, & you won't get it all anyway.
