The Brit standard was 3" at 100 yards. Each maker differed but that was the standard for the better quality makers. I agree with you that a Double should be expected to do that. I can no longer see well enough to hold to this so I've changed my personal standard to 2" at 50 yards.

I know and have known more than a few shooters that could easily hold inside 3" at 100 yards and a couple that were better than that. The most accurate Double I have ever seen is a 475#2 H&H that was rebored in Austria from a 450 3.25. It held 8 shots inside 2" on one occasion and will hold 4 shots inside 3" all of the time. It is currently owned by a friend and he refuses to part with it at a price I consider fair.

I think the range of a correctly regulated Double is only limited by the eye sight and ability of the shooter. Don't forget the Brits were shooting 1000 yards with open sights in the 1870's and doing pretty damned well.