I have used regular rifle scopes on muzzleloaders for many years, had to because when I started using scopes on 'em, there were no scopes made specifically for muzzleloaders, or shotguns either, for that matter. If I recall, the first scope I ever used on a muzzleloader was a Weaver J2.5. It, and most of those used subsequently worked out just fine. You have to wash the BP fouling off of the tubes after each shoot, and mount them so that powder and primer flash doesn't get onto the ocular lenses. But, there is nothing other than powder fouling which would tend to subject your scope to more stress when it is on a muzzleloader. Any scope which will stand up to the recoil of a .300 Win. Mag will do OK on any muzzleloader up to .54 caliber. This is especially true of round-ball guns, which have little recoil compared to modern rifles. Slug shotguns are a different matter, their recoil can be fierce. I suggest low-powered scopes, the idea (and appearance) of say a 3X-9X with big objective lens on a muzzleloader is ludicrous. You need a good field of view, not high magnification!! While parallax can be a consideration, there would not be enough in a 2.5X regular rifle scope to cause you to miss the vitals of a deer at less than 100 yards. Anyone that says you have to have a specially-made scope on a muzzleloader just doesn't have enough experience in this subject area to have a valid opinion.

Larry