But if you're holding it correctly the recoil isn't really anymore than using a shotgun/flat stock.
I'd read all the stories about how bad crescent butt plates where to shoot and I was seriously afraid to fire my first one until I had a recoil pad arrive from the states. So I ended up having this hawken sitting there for a month without firing it. It arrived, I put on the recoil pad and added extra foam and braced myself for the first shot. "Pop", jeez that wasn't too bad maybe I'll try it without the pad. I held the gun as it felt natural to do so and was blown away by how little recoil there was.
I once saw photos of a guy with cuts and a big bruise in the middle of his shoulder where you'd hold a modern rifle. I tried to replicate the hold with my hawken and it was so unnatural and uncomfortable I couldn't understand how someone would even be able to lign up on a target well enough to commit to pulling the trigger let alone thinking it wouldn't beat the shine out of them.
Just hold it right and even stout loads are fine.
I've been shooting since I was about ten.... that would have been 1966. Even if you hold it correctly, and even if it
isn't a crescent butt plate, just a flat or slightly curved steel/plastic butt plate, you
will get punished by heavy loads. Now with muzzleloaders, you have two variables; weight of the projectile and the charge weight. Both have an impact on recoil.
If you are shooting a heavy conical bullet with a heavy charge of powder, expect some punishment. A .45 caliber PRB with a 50 grain powder charge is quite manageable.
Recoil pads do help... which is why they were invented in the first place. That said, in order to use one on a muzzleloader, you would have to cut the stock to maintain the length of pull. I'm not willing to do that. You could also get yourself a Past Recoil Shield, which straps onto your body and has the same effect. This works fine in the warm months of summer when you might be wearing only a T-shirt, but really, it isn't very necessary in the winter when you are wearing a heavy hunting coat.
A shotgun with a flat stock, if you are shooting slugs, dishes out more recoil than most of the big African calibers... especially if it is a light fowling piece or something like an Ithaca Model 37.
There is no getting around Newton's law of motion (equal and opposite reaction....) You can hold it correctly, which according to experts, is to brace the crescent butt plate on your arm, not your shoulder, but take a look at the bruise an earlier poster put up. Holding it properly will not always mitigate the effects of recoil all that much.
So, in a nutshell, if you are hunting
Tyrannosaurus rex and plan to use an effective load, you are gonna get clobbered by recoil. If you are hunting Bambi, then you don't need a 600 grain No Excuses bullet with 120 grains of black powder.
Then you have the human factor. I had a friend once that actually shot pretty well... with an M16. Thing is, she stood 5' tall and weighed about 90 pounds dripping wet. So she had a pretty light frame. A "stout load" in a .54 caliber muzzleloader would put her on her backside if she were to shoot off-hand, and on a bench, I would not be surprised to see damage to her skeletal structure.
This one of the reasons most small-framed people don't shoot heavy recoilers.
Heaviest powder charges I shoot are generally around 70 grains and most projectiles are just patched round balls. I suppose I could load up some heavy conicals with 90 grains or more of powder, but there is no need for that. My water-filled plastic milk jugs explode just fine with the round balls. If I decide to hunt whitetail deer next year... doubtful... then I will be using a JHP pistol bullet inside an unmentionable most likely... or maybe a .530 PRB... neither of which would be at all punishing and both of which would be quite effective if placed properly.
Some people are simply more sensitive to recoil than others, and while training can be helpful, it still cannot eliminate the laws of physics.
There, done ranting now.