Hey Hey Deacon
Based on your measurements, My opine would be for .358 Bullets, which for BP would fill out the Grooves (as it should) when cast soft. I think the other guys (above) suggested the same dimensions also. Have fun with that beastie
Heading out, but time for one quick answer. Coffin Maker is absolutely right.
You lucked out in the greater sense because you now have a much greater choice of ready made bullets. Ø.358 is what you need. But as Coffin Maker said try to get SOFT lead bullets, not Wheel weights or even range scrap which tend towards 12 BHN, get almost pure lead, you can get it. If you need the name of someone who sell honest soft lead on Ebay I can get it for you. He uses roof lead and I measured it as 4-5 Bhn. I was hoping you didn't have one with the Ø.361 throat openings.
~Mako
So get any Ø.358 you want including molds.