I see two problems with this idea
first, the small size cap have no plain side but is ribbed so there is allways a source for gas escape

second, the thin copper cap on a muzzle loader will rupture when firing
ok, the cap having no protection on the nipple of the muzzle loader but the chance is imho, very high the primer will blowing out gas

with black powder load and a hammer gun pressure will be never high and the gas will not reach the locks but it seems you get pierced primers all the time.

maybe trying this idea to see how it works in practise