Bob,
I have generally reached the same conclusion regarding the groove depth. The gun shoots fairly well with round balls but let there be no doubt, there is no way to seal that gap with patches. It makes me wonder if Mr. Reinhard was thinking the same as others in that era regarding target gun barrels, that being to construct them with intention of having them rebored after shooting them a bit. Billinghurst and his peers did so frequently from what I've read.
The late Richard Hicks made a mould for a Lewis style picket bullet for the gun but there was no success to be had. After a dozen or so attempts I gave that up. Mostly they landed sideways.
I have no doubt it was intended for picket style bullet due to the twist, the era of it's manufacture and the muzzle being configured for a bullet starter. Not to say I'd rely on any one of those factors, but with all things considered I cannot see it any other way.
Thanks for your suggestion!
If I were going to shoot it,I'd try to find someone to dress and concave the lands and maybe,if needed
clean the grooves.If possible,maybe a .405 land diameter using a soft cast 41 magnum bullet loaded thru a new guide starter would work.
N.G.Whitmore made a picket gun for General Grant that when tested by him (Whitmore)at 110 yards
put ten shots on the lid of a percussion cap box.This was done with a pin head front sight and a tang
mounted peep sight.Whitmore made 12 groove gain twist barrels that out shot every thing of their time
and I think if someone would resurrect this idea we might all be surprised.
Major Roberts said a man he knew had a Whitmore bullet gun that outshot everything and was finally
barred as being unfair competition.Something to think about.
Bob Roller