Grener's book has a target fired at 50yards from an 8 bore ball gun. The wad under the ball in loaded ammo, has a cup - to centre the ball (just like the Swedish cup wad form a shot load), just as the plastic wads I used in the 12 bore were used. I had forgotten where the idea came to me long ago to use the plastic cups off an AA wad, and just now, remembered the picture of the 8 bore 'shell' from WW Greener's book. That was it.

Cup wads can be made of many materials, ie: Donnaconna, the fiberboard (black one side) placed in walls for sound deadening, and on the outside of new houses before the vinyl or aluminum siding. Anyway, boring/drilling/punching a hole in the centre of this 1/2" or thicker material, will give you a wad that will centre the ball, rather than having it bounce down the bore, if slightly undersized. Think of what might work for whatever bore gun you are loading balls for.