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.
