Good question, Curl. For shooting in a choked gun, the ball must be under choke size. That is .690" in a normal full choked 12 bore. The extra tight chokes as made for shooting turkey's or steel shot may be tighter yet, I don't know, haven't measured any of them.
: I have only used plastic cases for my loads. I'm assuming the brass shells are thinner, and have more room. This will changed the wads needed, etc. Trackofthewolf.com has all the different gauges of wads available.
; My idea of the lower plastic cup for centering the ball in the barrel was a good one, however, other articles can be used. Use your imagination.
: A wad punch, like a 1/2" or 3/8 arch punch, chucked in an electirc drill and run through a fiber wad, or several 1/4" card wads, will make a centering device that will hold the ball in the centre of the bore. Load the card wad or wads on the powder, then or holed fiber or card wads, set the ball in the depression and it will be held in the centre of the bore for a perfect launch. This centering device is only to prevent the ball from bouncing off the sides of the bore on it's trip out to the muzzle. This was why the 18th century muskets were innaccurate and why the pumpkin ball 12 bore round ball loads of the pre-foster slug era were also inaccurate.
: Something must be used to centre the ball - this is only limited by your imagination. As well, for filling in the centre of shot patterns from cylinder bored gun, the holed wads will keep extra pellets in the centre of the pattern for you. Francis Sell, loaded his heavy 20 bore goose loads like this in the 1960's. Nothing new, just old ideas used for different applications.