The undersized, but cloth patched balls were more accurate than the larger balls that engraved in the rifling.(16 bore 2 5/8" brass from Buffalo Arms)

30 years ago, I shot round balls from a 12 bore double, using the base cup gas check cut from used WAA12 (white) trap wads. The gas check was was placed under the ball, cup-up to hold the ball in the centre of the bore for it's trip down the tube. They shot very well for me. Friends of mine wanted to carry these loads with them while duck hunting, just in case a moose happened by- this happened often.

Because they were using full choked duck guns, I suggested cloth patching the ball with cloth thick enough that they could 'just' push it through the choke, and load that in a shotshell, using the highest speed 1 1/8ounce load they could find in their shotgun loading manuals. They did this and reported excellent results on moose to 50yards with impressive penetration, from their pump action duck guns.

If using black powder, a decent 12 bore load would be 5 drams + the requisite wads to get the column height correct for either roll or folded crimp. Lyman shotshell loading books have data.

Get the wad column right, then lay the cloth patch over the ctg. mouth and press in the ball. I use denim as it is very tough and it's available in many different thicknesses, from 6 ounce to 12 ounce, about .010" to .030".