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. I’m also thinking those #s are for maximum bore size. Not sure what ‘cylinder’ is at the muzzle on the 835, but it’s probably closer to 12ga than 10ga.
SRM Terror tubes use some type of geometry to go really tight… down to .655, and they work great on Invector Plus and other systems.
