After some extended success ,I had problems for some time with my 80's FT rig,HW77 25 mm dia piston,80's final tune used at Marklyate, mk1 Ken turner trigger in a John Welham stock.No synthetics,dry slide and Abbey lubs.
Aim seemed to be high on simple 40's shots and beyond after zero on 35 spot on.All movement on verticle.
Ensured the front attachment screws were evenly spaced and torqued,and space/gap at rear of trigger block was up tight.
Don't use bear trap,but guide is machined into trigger block of silver steel and sized to give grip at rest and loose on compression against rotary needle bearing.
It took me a month to find the fault,shown up on my pullover by wear. The but pad where I shouldered it on hold had moved by a small amount.Hardly noticeable but when corrected the consistancy of hold brought me back into the ratings again.
We had an example of the welds on the barrel to body anchorage breaking on a HW springer at the club.Whitness marks showed the barrel had moved slightly out but accuracy hadn't been affected,compared to later repair.
My problems were slight hold change which was difficult to see but had a real difference on the range.
Fighter against the 'Dark Arts'
Dark keeps moving in on the day.
Steadily going downhill faster!