I think the .835 has a bore diameter of .777, which is actually bigger than a 10 gauge at .775. Choking that down to .680 (.097) is quite a step down.
Those and Browning overbore stuff have barrels that gradually come back to a nominal size for their choke system.
Maybe some do, but the Browning does not. For instance the Browning Invector Plus bore diameter is .745. I just measured a Calson's extended choke I have for that barrel that is labeld .665. The inside diameter at the back end is exactly .745. The diameter at the muzzle tip is .663. So it appears the squeeze down occurs solely in the choke.
.729 is English ‘nominal’, and Browning’s usually run closer to .742 (their back bore spec) on invector plus….Benelli stuff is closer to .720. Fabarms had various ‘stages’ of bore step down. The Inv Plus system is one, long parallel section, that tends to flare slightly out, right at the choke. The chokes for it have been figured out for a long time, by the manufacturers, with lots of shooter input.
Hastings WLII (IIRC) had a .745 bore and a very stiff barrel, with straight rifling. Terror .675 and .655 tubes are amazing in both systems. How the choke gets from bore dimension to final choke outlet is the thing that seems to make the most difference, not worrying over the #s so much. Counting the intervals, most of my extended, tight invector plus tubes are near 3” long.
