Update! We shot it today and had a great time.
We had wing musket caps, but they took a lot of force to seat all the way on the nipple. But they worked just fine.
Test load was 70 gr FFg behind a .58 patched round ball. No issues. After that, we were using 100 gr behind a Minie ball, and that stayed nearly minute-of-man at 300 yards. We probably put a couple dozen rounds through it this morning. What a great piece of history on the wall in The Armory.
A couple of the guys are experienced with black powder and muzzle-loaders, and so we had a black powder day at the range. The 1863 Springfield rifle was just as accurate at 50 yds on paper as their "modern" Hawkens repros.