I'm not a muzzle loader shooter but I think you received some bad advice. I know I wouldn't put an airgun scope on any of my centerfire or even rimfire rifles.
If you think about the physics involved in a spring airgun, scopes are designed for the 2-way recoil generated by the spring and have their optics anchored for recoil going both ways. There are different levels of airgun scopes designed to handle different levels of spring recoil (some airguns have more) and you usually pay more for increased capability.
I think you ought to take the scope back to that gun shop and tell them it didn't work as advised. IMHO.
