Main charge generally is in something like a 28-bore shotgun shell with no shot. Easily replaced or reloaded.
No reason you couldn't make increment charges to fit between the fins. If I were to do it, I think I would use nitrated paper (as for combustible cartridges), load them up with Unique or something close to it, glue them shut and shellac them to waterproof.
Just put as many as you need between the fins, elastic band to keep them in place, and go play.
You have to be really careful what you are doing. It is REALLY easy to overcharge a mortar....... and blow the bottom 4 inches out of the tube..... sideways.
Before you do ANYTHING, find out the original charge levels and pressures, then approach mil spec cautiously if at all. If the Army wiped out a crew in combat they could always write it off to enemy action. You don't have that luxury..... unless you have a LOT of expendable friends.
Come to think of it.......
Hmmmmmmmm...........................
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