This 'ball' holder is made from a steel shot wad. The inside base of the steel shot plastic wad is nicely cupped, with an expanding base powder-cup on the bottom. These will hold the ball in the middle of the bore in a smoothbore for it's trip down the barrel, as well as taking the rifling in a rifled gun. The best part of this, is you can use balls from 16 bore, to 12 bore in these cups, in a 12 gauge gun, with also varies the weight, recoil and velocity for regulation possibilities, form 16 bore at ,662" up to .730" - or whatever your bore happens to be. Additional wads can be placed between this wad and the powder, depending on type of crimp and load used.

Here is a group, re-printed from Greener's book of a smooth 8 bore SXS shooting balls at 60yards.

Here's a break-down on ammo with an old Mossberg pump gun.