I wouldn't worry too much about it - the original bores and chambers are all over the place.
Use black powder and a paper wrapped bullet, and you'll be fine.

My 450 No.1 shells measure .484" at the neck, loaded with .450 bullets patched to around .462, with
Bertram cases.
The fired cases measure .487 at the mouth, with the neck increasing in size to .500 just above the
shoulder.
Such a chamber shape keeps the round centralised, but with the taper it's easy to load, even with
fouling present.

My rifle is .450 bore, with groove diameter approaching .469 - yes, the grooves are over .009 deep.