I've owned a couple of Bsf airguns a 55 and 70 and serviced and tuned quite a few always found they shot nicely at around 10.5 ftb with the standard leather washer and spring fitted with delrin guides and the right lubes etc fitted a synthetic piston head to a 55 I found it wasnt as good as the leather head its locktime was faster but harsher slight power increase but I eventually put the leather head back shot much sweeter,
But to be fair the Bsf was a strong well made gun but not a great gun not in the same class as HW Fwb or originals of the period.

Dave.