For what it's worth, I have a high grade 8 bore Scott single made in 1872. It is truly in mint condition. I found that it's chamber was actually oversize compared to American 8 gauges. I have to normally resize the head on industrial shells to make them work, but the Scott actually works perfectly with the industrial hulls. The bores, conversely, measure to true 8 gauge. I wonder sometimes if chambers may have varied a lot during that period. I have a P. Webley 8 double of the same period and it has a standard chamber.

Bores are different too. Parker used 11 bore barrels on their 12 gauge guns as a normal practice until the end of the century. That situation was confounded by the fact that early on they chambered for both paper and brass shells. None of their guns are marked. Although they made a veritable hand full of documented 11 gauge guns, you see magazine articles and ads constantly that have discovered the latest "lost 11" based on a bore measurement and the fact that a particular cartridge won't chamber or is loose. It was a pretty dynamic period.