Thanks for the note. I've got the moulds for the 12 and 20 ga. in Lyman. A pain to cast - have to heat the plug with a tourch to get it to fill out. Accurate when paper patched up to bore size. Too big to put inside a shot cup and small enough to slide through a full choke leaves much to much clearance.
So the principles are clear - minimum clearance in a smooth bore or fitted to a rifled bore.
Now, if I can just find the mould. THere are huge ones made for the Nitro express rifles - but nothing I've seen yet for a good fit to 10ga.
And then the chokes- someone ought to be making them out there.