Yeah, I put down some rifle criteria first, then worried about caliber. I wanted a long rifle at least 38" barrel, and swamped, a single trigger, not much hardware and no carving. I was looking for a simple rifle that might've been carried by a market hunter. Patchbox could be wooden or not present at all, and brass hardware for the little bit of hardware that was there. As I wanted to hunt deer, it had to be at least .45 caliber, and the fit of the barrel and the lock to the mortise for me was crucial. Also It had to be priced at under $1000.00 as that was all I had.
I found a used rifle that met all those requirements in .54. Calibers .45, .50. and .54 can usually be found in round ball in gunshops not heavily dedicated to BP shooters, but if I had found the rifle in .58 I would have bought a mold for it as my next objective.
In your case as the folks have suggested .45 - .50 will suit your needs for punching paper, and you don't have to go swamped barrel as I did for I was concerned with carrying it all day while hunting, while for range use only that's not an issue.
LD