This is the method I use for centering slightly undersized round balls in the bores. It also works for shooting undersized balls through any choke, yet perfectly centres the ball in the bore for best accuracy. The gas-check portion of the shot cup will spin the ball in a rifled arm as well so it's not just for smoothbores.
: The mould is a Jef Tanner mould, with Lyman handles. He will make any size for around $30.00 last price I've seen.
; The steel shot wad has a nice cup facing up, with a squarish gas check powder side. The trap wads, when fingers are cut off, are flat. When using black powder, always protect the plastic with a hard card wad, so the plastic doesn't melt onto the bore. Shown in the picture are three sizes of balls for shooting in a 12's large and small.- .684" for use in full chokes, .715" for use in Modified and .742" for over size and .75 cal.(bare) and .77 cal.(patched) muskets. I currently am using a .725" mould for my 12 bore, cylinder bore loads.