Hi Dru - Aha! A Steinbusch (probably spelled that wrong). Historically of course, they were firing a cut stone projectle, that would be lighter than a cast iron one. They would fit a sabot between the charge and the projectile because of 1. crappy expansion rates of Medieval powder and 2. resulting gas escape from the irregular fit of the projectile.
Like most modern gunners, I assume your charge will be in a tinfoil cartridge, and you can tailor the form for the chamber itself. Most competative shooters use a milled lead projectile - I don't know if you want to pop things downrange or not, but the modern methodologies would allow for a tighter projectile fit, and remove the need for a sabot.
The mortar crowd of the AAA is usually unfortunatly the dregs of the artillery world. I have personally witnessed boobs in the mortar competitions stick their heads over the muzzle to "see" what was wrong when they got a hang fire. Getting liquored up in there beach chairs whilst they sit outside their convieniently parked motor home on the range is an awe inspiring sight, and a sure-fired enthusiasim builder to the serious cannoneer. I wouldn't trust them (on average) to competently band-aid a scratch, let alone give good advice on the subject to hand.
You might need a cardboard sabot to fire an effective blank - I don't know.
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Bob R.