Keith, For my Iron Napoleon I tape the powder bag to the ball with masking tape. This keeps the ball next to the powder. I have tried sabots with mixed results.

I would recommend that you attend the N-SSA Nationals for their live fire artillery match. I would also recommend that you use N-SSA artillery loads and shoot at 100 yards to start with. Gain some experience and then shoot at 200 yards. Just make sure that you have a safe place to shoot. Even with reduced loads projectiles travel a long way.