Humpty & PM,
yes H&H is a way to go, the problem is that the gun's bores have straight rifling from the forcing cone onwards up to three inch from the muzzle. In addition the bores are .730" only. One possible way to go would be to backbore to .735" (the H&H standard), leaving the last three inch of twisted rifling. Barrel wall minimun is now .040" and measures .055" min. in the "choke" area. .
As to the rifling, yes I may call it invisible, it has six grooves. The are no chokes at all, the only difference there is the twist and barrel wall thickness. Hope the British audience may have something to say on how they are currenlty using these ball and shot guns.
On another issue, yes it is surpring the unusual chamber length, it may have been enlarged.
EJSXS
Your bore measurements sound mint
DO NOT ALTER THEM IN ANY WAY The invisible rifled guns' bores are tighter than rifled choke guns. I suspect your chambers were enlarged at some point in time.
