I shoot an unregulated USFT (Hunter #150) in 25 meter heavy varmint (20 FPE max) benchrest.
I can regularly shoot 247+ cards but occasionally I’ll get shots that are 2 to 3 mm high or low which prevent scoring a 250. I’ve traced the problem to variations in muzzle velocity. The POI on the y-axis on average goes up/down 0.17 mm for each FPS swing in muzzle velocity. (This is in line with predictions by Strelok and Chairgun.) Even with tethering to keep reservoir pressure constant, across 60 shots, I can often see an extreme spread of 35 FPS (with SD of 8-10) which implies a y-axis range of 6 mm which is not good for scores.
I’ve tried changing poppets and hammer springs, but I don’t get much change in FPS spread. I wonder if the issue is friction in the trigger mechanism or Hammer spring guide.
This leads me to several questions:
Hoping some of you USFT folks can provide guidance on this problem.
Follow-up: Tim McMurray got back to me. To minimize ES, he recommends maximizing hammer spring tension and using the transfer port restrictor to adjust to desired muzzle velocity. Less hammer spring travel reduces the effect of drag which can vary from shot to shot.
I followed his guidance and based on tethered shot strings of 100+ shots, the standard deviation of muzzle velocity fell from 8.1 FPS with the hammer spring adjuster set 6 turns back from max and no transfer port restrictor, to 4.6 FPS with the hammer spring adjuster set 1 turn from max and the transfer port restrictor closed enough to achieve target velocity. Spreads have tightened significantly as a result of more consistent muzzle velocities.
I can regularly shoot 247+ cards but occasionally I’ll get shots that are 2 to 3 mm high or low which prevent scoring a 250. I’ve traced the problem to variations in muzzle velocity. The POI on the y-axis on average goes up/down 0.17 mm for each FPS swing in muzzle velocity. (This is in line with predictions by Strelok and Chairgun.) Even with tethering to keep reservoir pressure constant, across 60 shots, I can often see an extreme spread of 35 FPS (with SD of 8-10) which implies a y-axis range of 6 mm which is not good for scores.
I’ve tried changing poppets and hammer springs, but I don’t get much change in FPS spread. I wonder if the issue is friction in the trigger mechanism or Hammer spring guide.
This leads me to several questions:
1) What sort of ES and SDs do USFT owners get with their guns, regulated and unregulated/tethered?
2) What ideas do you have for reducing spread?
3) If this involves dissembling and reassembling the trigger section, can you provide any procedure for doing this, or at least any “watchouts” for this? I have not tried to remove the pins holding the trigger assembly in the gun for fear of not being able to reassemble it.
Hoping some of you USFT folks can provide guidance on this problem.
Follow-up: Tim McMurray got back to me. To minimize ES, he recommends maximizing hammer spring tension and using the transfer port restrictor to adjust to desired muzzle velocity. Less hammer spring travel reduces the effect of drag which can vary from shot to shot.
I followed his guidance and based on tethered shot strings of 100+ shots, the standard deviation of muzzle velocity fell from 8.1 FPS with the hammer spring adjuster set 6 turns back from max and no transfer port restrictor, to 4.6 FPS with the hammer spring adjuster set 1 turn from max and the transfer port restrictor closed enough to achieve target velocity. Spreads have tightened significantly as a result of more consistent muzzle velocities.
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