what choke do u have? my 20ga has a mod screw in choke. i use the wads (overshot-cushion-ball-overpowder) and the bare ball is a nice tight fit through the choke. i use .610's and the choke measures .608.
i think it helps to stabilize the ball as it exits, because it doesnt seem to matter what powder charge i use or what kinda wads i put in as long as there is a cushion wad right beneath the ball to keep it centered. its an extremely acurate smoothbore, and the ball always hits perpendicular to the ring left by the choke.
also if u want to use some plastic shot cups, just drop a smaller ball in the cup after u have it in past the choke. i use .530's in the WAA20 wads and it works pretty slick. put an o/p card, or 2, down and 1/2 a cushion or a wonderwad beneath the shot cup and u wont have any trouble with plastic in the barrel. i bet .650's would be a good fit in ur 12 with WAA12 or FIG8's.....