The groove to groove I meant, was largest size, corner of the flat & notch to corner of the flat and notch.

The breech picture shows slower rifling what what they look like in the first muzzle-shot. This is good.

Due to the style, the difference from smallest mesurement, tops of the flats across to depth of corners looks like fair depth. It's the difference from minimum to maximum that's important.

I would think that a 5 dram load should give around 1,300fps at regulation, as Bob found in his gun.

If the barrel will allow a true 10 bore ball(.775"), in pure lead, they'll be about 700gr. of course. In WW alloy - about 670gr. I'd think something like 50:50 WW/Pure would be about perfect for penetration and easier on the barrels than if harder. Shooting ability will, of course show what's needed.

The chambers appear to be cut for a fairly thick case. Lucky, but brass would have been nice, just the same.