Working from memory here, but I believe when Ithaca was researching Hawken rifles, in order to design their own, they found historical references indicating many of the originals shot a “half ounce ball,” which would measure .526”. They may have confused ball size with bore size, or maybe didn’t realize patched round balls are under bore size. In any event, the Ithaca Hawken rifles ended up being made in approximately .53 caliber. Mould makers responded by creating .520” round ball moulds, and Hornady (not sure about Speer) still makes swaged round balls in .520”.
Navy Arms and then Pedersoli acquired the tooling to make this rifle when Ithaca discontinued it.
So, as I understand it, the older Pedersoli Hawken reproductions were about .53 caliber, and required a .520” ball.
Undersized bores are not uncommon. I found an original GRRW barrel in “.54” caliber and had it sent to John Bergmann to build a Hawken around it. The first time I tried loading it, I used a .528” ball and ended up hammering it down with a mallet. I fortunately had some .520” balls, which loaded easily with just the ramrod and shot well, so those are the ones I use,
Best regards,
Notchy Bob