All the answers above are correct,,but Shorty touched on the meat of the issue,,
,, it's seems folks want to use a musket cap because they feel it will deliver a hotter mor sure fire ignition. And of course,, they do.
But musket caps are made for muskets,large open bore long guns and short rifled pistole,,generally moderate to heavy loads of powder and projectile required for short range accuracy aka;50yrds and under.
There is an issuse about longer range,, higher pressure loads that is sum tymes refered to as "gas cutting",,and it's the reason #11 nipp's have a small, little, teeny, tinny hole in the bottom and why theres are several "paten" on different nipp chamber profiles.
Think about it,,your touchin off an explosive in a confined area,( a "caustic" explosive at that) it's going to try to find the path of least resistance..( it should be pushing everything down the bore ,right?). Well some of that pressure wants to go out that litte hole in the back of the gun, the nipple. That's why the hammer and lock spring and all it's components need to be cared for also, if the hammer won't hold down that cap,,then gasses are released rearward and there is no consistant pressure forcing the projectile down range,,aka ("jeez"!,I wish I could get better accuracy.)
With proper care and cleaning,,the #11 nipple and cap, can and will provide reliable ignition and shot to shot accuracy..Sure you can get musket nipps' to fit just about anything,,bend the hammer,,grind the nipp, and/or get everything to fit proper so it drops just right,,. Why? I mean, why do it??