I've had two shotguns with porting, a B325 12g and my current B525 20g.
In all the shooting I've done with both guns, I could never tell any advantage. I'm not saying it doen't work, just that I'm not a 'sensitive guy'.
I ported a 870 trap barrel for a guy that just had to have it done. I advised against it, but he would have either me or some butcher with a hand drill do it. So, I did it by copying my Browning 325 pattern, using a dividing head in my Bridgeport, and used a brand new carbide endmill to reduce the burr, then lightly honed it. The customer was very happy, even after he had shot it. I couldn't figure out why a trap gun needed recoil/muzzle jump reduction.
In theory, it reduces both recoil and muzzle jump. This is theoretically due to redirecting the gas ejecta up and sideways, lowering the forward ejecta. The effect is most dramatic on high pressure handgun and rifle cartridges where gas/powder ejecta vs projectile weight is closer. The unlimited combat guns with compensators and the big rifles show pretty obvious recoil reduction benefit from porting/muzzle breaks due to this close ratio of gas ejecta vs. projectile weight. A shotgun has a much lower pressure and gas/powder ejecta weight compared to the projectile and therefore the effect is much less dramatic...so much so that I could never feel the effects.
