I have heard that the Bisley Single-Sixes, at least the earlier ones, had a different bore diameter for .22LR only but I don't know if that's so or not. I have one of the early Bisley models with fixed rear sight and it is far and away more accurate than any of my other 5 convertible Single-Sixes. From a knee rested position, it is fairly rare that I can't knock down 5 standard rimfire rifle silhouette turkeys at 50 yards with one cylinder full with its favorite ammo. I think that's pretty decent performance for a fixed sight 6 1/2" barrel revolver. Fortunately it's favorite ammo also seems to shoot to the same POH/POI at 50 yards.
What always struck me as odd on those early Bisley single sixes is the fixed rear sight only adjustable for windage, and then by the difficult and imprecise method of tapping it. Fully adjustable sights seem to make so much more sense on a "target" .22, particularly since .22s are so ammo sensitive and group size/POI can vary greatly.