I want to buy my husband a scope for his muzzle loader for Christmas, are most compatible with all? I don't know if I need specifics on the muzzle loader, or can just go buy any muzzle loader scope?
Look on the top of the barrel. If you see 4 small screws toward the rear of the barrel this is for mounting the rings that hold the scope. If not a gunsmith could drill and tap the holes. Any standard scope can then be put on. There are also mounts made for muzzle loaders that do not require holes, a well equipped muzzle loading shop could furnish these if there is one in your area.
Most muzzle loaders will need to be drilled and tapped, so you can attach the scope rings, that runs about $35-$50 here. Muzzle loader scopes sometimes will have more horizontal cross hairs. But generally a normal rifle scope can be used without any problem. Your local gun store will be able to tell you everything you need to know, if you know the model/brand of your husbands muzzle loader.
ONE item I noticed that others missed. Be sure scopes are legal for muzzleloaders where and WHEN he hunts. In SOME states, scopes and/or sabot bullets can NOT be used EXCEPT during REGULAR rifle season. (THIS applies in states that have a primitive weapon season that is separate.)
You can use any scope out there. Search the bargain cave section of Cabela's for starters. You can spend boo coo dollars for a scope and they're worth it but a bargain scope is all you will need for the muzzy. I've got a Tasco and a Simmons on my two muzzleloaders and they always hit where you aim them. Good luck and have fun!