It decreases perceived recoil. I am not an engineer, but I can tell you that it does make a difference. Lengthening the forcing cones also makes a difference and that is something which makes perfect sense from a pressure standpoint.
The size, shape, and direction of the ports all makes a difference. Each company has a certain configuration for their ports, and they can all explain to you why their mouse trap is the best. In my experience, the Angleport work seems to do a better job at reducing recoil than the others. Magnaport does quite a job as well. Seminole does great work, but an Angleport worked gun just like it will have less recoil (at least that has been my experience).
I have my Trap Doubles gun ported for a quicker follow up (2nd shot). And it does decrease recoil. We shoot modified 1187,s with ported and pinned barrels. But the key feature we add is a high adjustable comb. The adjustable comb rasies the centerline of the eye to be even with the centerline of the scope. The gun fit and porting really reduce recoil.
If you want to reduce recoil, I would go to a gas operated semi-auto. The extra weight and gas operation will help some. You have to realize slug guns kick like heck and just learn to absorb the recoil- don't try to fight it. If you're going to shoot with a scope, get one with at least 4 inches of eye relief and make sure to mount it the right distance from your eye.
Hey Beefstick, I shoot a lot, both in competition and hunting so learning how to absorb recoil is not the issue. The question was about Muzzle brakes and porting and there advantages. Overall for most shooters proper gun fit is problem for excessive recoil. Anyway the only time you notice it is shooting at the bench, when i am shooting at game I don't even notice the recoil. And i will tell you the brake on my 300 mag changed the way that gun shoots. The gun was fitted correctly, but still kicked on the bench. After the brake was installed it shoots like a different gun, really reduced recoil. So there you have it, proper gun fit and a brake or porting will reduce recoil.
I can handle the recoil just fine. I have no problem sitting at the range running slugs through gun all day, recoil has never bothered me be it a high power, shotgun, or muzzleloader. My biggest concern is muzzle jump and speeding up my recovery time for follow up shots. In my area during slug season there are a lot of drives and hunting pressure that keeps the deer moving at a pretty good pace through some thick timber so its not always a one shot one kill type scenario. I do shoot a gas operated automatic with a busnell halosight and a raised comb stock to help keep my cheek welded solid and in the right place, but even with that set up its hard to recover and get back on target when the target is moving full speed ahead and only giving you small windows to shoot through and your throwing huge chunks of lead like the 3" buckhammers that I shoot