Rob I have measured a lot of BSA production rifle barrel twist rates over the years and only ever seen them in the 18.5-19" range, makes me wonder whether yours is actually a bsa barrel?
I am dubious about the claimed origin when barrel blanks are unmarked, I recently fitted one which was bought as an HW100 .22 barrel blank, but it had the wrong number of lands!
I get the CZ barrels at 19" twist too, I have only measured those factory fitted to the S200.
Some I can add are the Barley twist LG100 barrel I had which measured at 15.5" twist rate
Anschutz 8002 S2 barrel which has a 19" twist rate too. Lots of rubbish around the net about these not being suitable for 12 ft/lb due to the twist rate being designed for 10m when actually they similar to many .177 barrels and work very well in a UK converted rifle.
HFT, for people with only 2 buttucks!
VooDoo TwoTwo The next chapter.