Dingelfutz,
It has been a pleasure reading your Posts in response to Mike's questions. I wish that it was possible to meet you and talk about this game in detail!
I will only add a couple of general type comments.
I hit a brick wall in my attempts to have a set of 20ga barrels jug choked. I was told by several in that business that the barrels would have to be seperated and re-regulated after being choked and rejoined.High $$$! My problem was solved by having a friend who is a barrel maker make a set of 16ga barrels choked before joining.I had the right barrel choked full, and the left barrel cyl. It is easier to reach up and back the right hammer than it is the left, and that's the reason I had the change from the traditional left barrel choked format.
I do have a jug choke in a New England Fowler with a 51 inch barrel. The piece patterns very well, and the jug choke does not affect patched round balls.
The fire hazard being greater with a ML'r, I'm not sure about. I've never had a bad experience with any of my activity loading in the traditional way with OP,cushion and OS cards. With patched round balls out of my flintlock rifles I frequently have patches smoulder, but only those that I dry lubed with water soluable oil and only on days with very low humidity. Overall I don't think the fire potential with a conventionally loaded ML shotgun would be very risky.
Mike, you'll have trouble finding 20ga cast round balls, and when you find them the price will be high. Best plan to bite the bullet and spend another $100. and get Ray Rapine to make a mould. On second thought, if you're in California you may be restricted from using lead! You may have to use ball bearings!