Thanks Gatsby - that is very close to what I surmised.

Excellent - my Mossy 835 rifled 24" bl. is .724", groove to groove. The 30" smooth bore is simply oversize at .735".

With Herco powder, my only test so far, I handles the undersized .710" (Tanner Mold) round balls beautifully even though it has the rather 'fast' 35" twist.

I need to test some heavier .724" balls (Tanner Mould) I cast some time ago, with Longshot and Steel powders. I should be able to get close to 2,000fps with "Steel" - I think, we'll see.

Loading somewhat undersized round balls inside a shotshell, but patched in cloth to take up the windage has worked well for me and also for my 'local' friends - it is something that should/could be considered as there is no leading. The ball itself does not contact the rifling, but is spun by it as if engraved. They are, of course, lighter than the Paradox or other elongated bullets - being close to the 3/4 rule? - a .715" ball weighing 545gr. in pure lead, whereas a .730" ball weighs 583gr.