Hello gatsby,
Bore is .807”-.808” 5” ahead of the chambers as close as I can measure the slug. Have not measured the actual choke.
Bore has possibly been honed at some time or another as the 10 bore 2/10 proof spec is .803” as far as I can determine from the old sources, and the bores are mirror bright. Unusual for a ~124 year old gun.

ADDED:
I am having Steve Brooks brooksmoulds.com make me an exact copy of the Fosbery HP bullet from the rather worn original H&H mould. Expected to cast out at ~.806” and ~875 grains. The Brooks moulds are some of the best, I have acouple of others to cast copies of proper weight .450 and .500 BPE hollow point bullets and they cast flawless bullets.
- Mike