I'm poking around with the 16 bore double rifles this weekend.
They share the same bore diameter, but the groove diameter in the short cased gun is larger. I think the rifling twist in that gun is slower too.
I want to set my 16b roundball mould up for the short case gun, I'm leaning to conicals for the Mahillon anyway.

Does anyone here shoot a bore rifle with roundball?
The twist rate in the short case gun is just under one in 100"
The brass for that gun is 30mm.
My sense is 2-3/4 to 3-1/2 drms of FF will do it for that gun and roundball.

Pure lead?
If not, what alloy?
I should know what I'm going to cast before I lap the roundball mould to size for the alloy I'd be casting.

--Tinker