3/4" ball bearing balls wouldnt be that difficult to find I guess , then make the bore 19,5 and you got both a good windage and a few tenths on the right side of the leagal border so it cant give you any problems at all .
you never know if some idiot cop would mic it to see if its leagal or not .
why break the law for a few tenths when you can use 3/4" ammo and be legal