With all due respect, I can't see any need for using a musket cap on a underhammer gun, unless you are trying to load it with something other than Black Powder. Is that what you are doing? Trying to ignite one of the substitute powders?
The very short distance between the percussion cap and the powder charge in the barrel in a direct fire ignition system like an underhammer( the other is the side hammer, a/k/a " Mule Ear ") requires only standard caps for ignition. Only when you are dealing with Slug Guns- Large caliber barrels shooting bullets, often heavy and paper patched, on top of huge charges of Black Powder is there a need for a hotter cap. The Slug Gun guys use a modern small pistol primer held in a special device that screws into the barrel in place of a standard nipple. They also have no use for a Large, Magnum Rifle Primer, the equivalence of that Musket Cap, in their underhammers guns.