RE: Porting?
Porting, as you have probably noticed, is not a very popular option among muzzleloading shooters. Over the past 30 or so odd years, I have been experimenting with porting and muzzle brakes on rifles, shotguns, and pistols. These devices work by re-directing the expended gasses of the powder either just before or just after the bullet has exited the barrel.
Before the bullet reaches the end of the barrel, there is reaction/counter reaction that occurs as a bullet of a certain weight travels, at a certain speed,down the barrel of a gun of certain weight. This is simple physics. Muzzle brakes and porting don't affect this part of the recoil equation. Object A moves forward at a certain speed, and object B moves rearward.
Once the bullet exits the crown of the barrel, a volume of high pressure gasses are released and create a "jet engine effect" by forcefully driving the gun back into your shoulder. This is the second part of the recoil equation; the partthat muzzle brakes and porting serve to mitigate.
The issue with conventional muzzle loaders and, to somewhat the same extent, with shotguns, is that a fairly heavy bullet being driven by a low pressure charge don't give the porting much to work with. About 75% of the recoil is being generated by the simple physics of the big bullet moving down the barrel; and the gas pressure is too low to give the porting much force to operate.
The opposite extreme would be a .257 Weatherby rifle. Not much bullet weight. Lots of high pressure gasses. Only about 30% of the recoil from this rifle would be generated by the bullet moving down the barrel; giving the muzzle brake lots of gas force to redirect. A muzzle brake can effectively cut the recoil of rifle like this by more than half. Noise from muzzle blast goes up a bunch as well.
Athough I haven't shot a muzzle loader with a brake or porting, I would suspect that the lighter sabot rounds driven by magnum charges might be worth the trouble. If your ears can stand the extra decibels. And, if you can choose, an expansion chambermuzzle brake not only works better than porting; but also gives you the option of removing it for cleaning.