I also have some Hopkins and Allen underhammers.
There is some variation in barrel diameters mine uses a 530 ball with a heavy ticking patch. It is one of the most accurate PRB guns I have owned.
The max load is 120 gr of Black or Pyrodex RS. I would not use anything else, especially if it is one of the originals.
Be sure to get nipples with the right thread as they are not 1/4 - 28 and if you force a 1/4 - 28 in it is going to weaken the threads and is liable to get blown back out.