dracb from BC,

The bare bullets are casting out at ~.501 for the 340g & ~.502 for the 440g so come to .507 and .508 respectively with a 2 wrap patch of onionskin paper.

The bores are 8 groove Henry type by Adams & Tate, actual "bore" at the tops of the lands at breech are ~.498" and muzzle ~.496"
The bottoms of the groove right adjacent to the lands are ~.511" at the muzzle.

Just like the originals I expect the 1:30 slightly "undersized" paper patch bullets will slug up to the barrel nicely. (Or at least they do in my other .500 3" BPE rifles).

I don't have a mould to cast suitable grease groove bullets for the .500 BPE types, "they" say that the Henry type rifling does best with paper patch anyway.
Interestingly enough I have a Cogswell & Harrison single shot in .500/.450 No1 Express with Henry type rifling that shoots astoundingly small groups with the RCBS .458 300g Gas Check bullet cast 1:20 so the conventional wisdom is not always accurate. I have never slugged the bore on that one.
- Mike