Throughout history 100 yards has been the standard distance for doublerifle regulation. Borerifles was however regulated at 50yards.

It seems the old trade of english gunbiulders set their own criterias of a acceptable regulation.

In the book "Sporting fire-arms for Bush and Jungle"(1884) by Captain F.F.R Burgess states in regards to distance and express-sights the only a 100yards was to be expected for regulation. For use at longer range only one barrel could be relied on.