I would first try pushing an oversize ball through a cheap Lee sizer die of a diameter to match the regular bullets you shoot in it, plus a couple thousandths. (Rather than go through the rigamarole of determining exact throat diameter, that would get you started.) Lee is pretty good about supplying odd diameters, or buy one off the shelf and hone it to size, no big trick. The case mouth could effect diameter but not enough so to swage a larger ball to desired size. If you were he'll bent on creating a projectile perfectly sized for the occasion.
The biggest trick I see in all this is getting the ball seated "straight" in the case. Ie: with either the paper patch or the swaged band square to the bore. Lopsidedness in that regard couldn't do any favors accuracy-wise.
Probably best bet is to prime/charge a fired unsized case and thumb seat a ball big enough to be a tight fit in the unsized case neck. Smear some lube on it, and see how it shoots. Might surprise you. Mold companies make a plethora of diameters for round balls.
Last edited by gnoahhh; 05/24/20.
