Hey Longknife - I like the idea of a bore rifle - of course. Couple thou. over groove diameter would be normally correct for cast bullets, unless you have a cup wad to take the rifling then release the bullet at the muzzle, spinning as it should. Lyman's Sabot mould does exactly this. They also have just such a mould in 20 bore, casting well over 300gr. in bullet weight to fit available plastic wads.

3 drams (82.0gr.) would be a good load in a 20 shotgun action.

I used 3 drams in my 2 - 1/2" 16 bore Husky of 1902, with under 14 bore balls in it's 13 bore (.705") groove diameter. The bore, was 15 at .675" - yes .015" rifling depth, each side.

The .690 balls were undersized, but the rifling was straight and with the wads needed to fill the empty space inside the hull, the shot quite well. Next time out, I tried 16 bore cloth patched balls (12 ounce denim) in the 2 1/2" cases and they worked just fine, accuracy wise and shot very clean was well.

I personally would stick with round ball - as a .615" is already at 350gr.

Depending on rifling depth, a .626" is 368.4gr. and .637" is 389gr. - all in pure lead, of course.

ALL THE BEST