The "ideal" size depends in part in what you are loading for - defense, deer, coyotes, etc. You want the shot sized to sit in even layers, or the forces involved will wedge them against the sides, with much greater deformation, and scrubbing on the bore if not protected. The proportions between the diameter of buckshot that would sit in even layers and the diameter of your bore (or the minimum of your choke, or inside the shotcup, or inside the shotcup passing through your choke, etc.) are:
- 2: 1/2 * the minimun diameter of bore, choke, or inside of shotcup
- 3: .4639 * the minimum diameter "
- 4: .4142 * the minimum diameter "
- 5(+1 in centre between layers): .36± * the minimum diameter "
- 7: 1/3 * the minimum diameter "
I suspect that normally, one would use something a hair smaller than the "exact fit" size to allow for imperfections, fouling, etc. I've never seen anyone calculate for layers of more than 7 - probably by then, one can just treat them as loose shot.
Regards,
Joel