When it comes to patched, round ball...,
You could use #1 Buckshot that was lead alloy for a .32 rifle, as it's .300, coupled with a patch. You might find that if you need a thin patch, that a little corn meal between the patched ball and the powder reduces burn-through. You should use a micrometer to double check the diameter when using the commercial, alloy shot.
(Some folks with .36 caliber rifles have had success with 00 Buckshot and a thick patch in their .36 rifles. As long as the patch and ball combo is tight enough to impart a spin, you should do fine)
Remington #1 Buckshot
Most of the folks doing pre-made buckshot in black powder, from the ones I've met and the ones I've spoken with online, are using the stuff in black powder revolvers. They are using 00 buckshot in their .32 pocket pistols (as it's .330 diameter). Further they cast their own out of a 00 Buckshot mold with pure lead, not commercial made buckshot. The commercially made stuff is normally hardened with antimony, so is a lead alloy, not meant to deform when launched from a modern shotgun, so it's too hard for the little loading lever on most .31 revolvers to swage into the revolver chamber.
The problem you might have would be finding a #1 buckshot mold, as the popular sizes are #00 buck, #0 buck, and #4 buck.
LD