I still think it works OneOunce ....although I think the reduction in recoil is minimal ....I still say it reduces muzzle jump ...(but I'll defer the physics equations to Zippy )...
I like ported barrels so much / I was delighted to see the barrel on my Browning BT-100 Pigeon grade ( single barrel Trap gun ) was ported as well
( of course the muzzle jump isn't really a big factor on a single barrel Trap gun / but it helps irritate the Trap shooters next to me / so that's a plus as well ...). Like eating a gallon of oysters, a guy can't have too many ported shotguns ...