The Mahillon runs 40+ mm brass.
This other sixteen bore double is of short case design, the brass (just about .028" or .75mm wall thickness) is only 30mm long. That's perfect room for 2-1/2 to 3 drams of black powder, a felt wad and card, and a roundball. That all comes together with plenty of room for jump to the rifling.

The Mahillon has normal case length for what it is (both of these guns are pinfire by the way...) but is running smallish for the bore designation balls.

I had a talk with Lewis Drake the other day about running the sixteen bore rifle and one of his comments was that running undersized balls -within reason- is fine, the real trick is to get good consistent gas seal. He says if I get good even gas seal that the road to a regulation load is to simply add black powder till it hits to the sights.
That is my goal at this time.
As it is, the mould I have throws perfect for groove diameter balls for the Mahillon. I'll just try them out on this other rifle and see how it goes.
Haven't had the bore rifles out to get running properly yet, hopefully this week the job will be in the bag.

What are you using for wads, grease, and gas seals?

--Tinker