We had a great run on this subject a few weeks ago. Methods of Ball loads in drillings and capes and chokes. Daryl_S of BC, Canada and beleg2 of Bahia Blanca, Argentina had great info.
Using the waste cut off shot cup gas checks from fired loads in a )(O configuration to center a choked bore size ball was a great idea I am going to try. I previously used felt wads with limited success with undesized ball loads. You might search for the past thread for these posts. Very helpful.