a muzzleloader scope and a rifle scope are basically the same thing. The thing you need to keep in mind is eye relief and I like to keep the scope short, so I do not have to remove the sights on the rifle.

As to some good scopes in 3-9x40 would depend on what you want to spend. I like around a 4" eye relief. I also like the short scopes around 11-1/2 inches. I personally like the Nikon Pro Sport but I use the 2-7x32mm and have been real impressed with it.

Good luck in your search for a scope.