I would refrain from steel ball bearings. They will score the bores unless well protected.
: In a modified or full choke, we found a .690" ball will pass a normal full choke or any lesser choke. Normal full choke is .690" and it is easy totest by dropping a ball down the bore.
: ww.trackofthewolf.com sells large round balls by the bag of 25, but they are pure lead, which mushroom. They are just fine for ungulates like deer, moose and elk.
; For increased penetration form a smooth bore 12, a mould from Lee or Lyman is the answer, in .690", .715". Jef Tanner in England will cut any size you want for about $30.00. With a mould, you can cast any hardness you want. With wheelweigths, you can drop them into a bucket of water Be very carefull not to get any droplets of water into the lead pot - it will explode molten lead everywhere. Dropping them into water, then retrieving them, and wating 12 hours, will result in hardened balls that will prenetrate like FMJ solids from a rifle. - no expansion at all.
; The trick to get RB's to shoot well, it getting them and holding them centred in the bore. Early round balls were left to bounce from side to side as they went down the bore, which causes innaccuray, just like the military muskets with paper ctgs. did in the 18th and 19th centuries. I use the plastic cup off the bottom of a retrieved trap shot wad form the trap range. They're free-and most are useable. Jusd tcut the top part off with a knife or side cutters. Alcan used to sell them separately called Air-wedge.
: I put one cup down on the powder to seal powder gasses, then a fibre wad, then another plastic cup up, which will centre the ball, then fold crimped over that. They worked splendidly. Use fibre wads and card wads to get the height correct for crimping, either a roll crimp or folded crimp. Lyman and Lee sell roll-crimpers that use a drill press.