I got into the shop today and made the valve mods. Close to what I expected, but I'm getting diminishing returns. I used a 0.028' thick spacer and trimmed the piston head back 0.030". That leaves enough shear stress area to balance with the higher tensile strength of the section at the piston o-ring groove.Average velocities:7mm, 48" barrel, 2650psi1030fps - 106.3gr BBT 250fpe1050fps - 96gr Bowman 235fpe257, 36" barrel, 2650psi1050fps - 71.5gr, 257420BT 175fpeThe 1030fps 7mm does have more recoil. 300m wind drift is only 3% more than the minimum drift, which occurs around 940fps.
I've done a little more tuning. The numbers I have been getting have been with slugs sized for maximum velocity. The last batch was sized to the groove diameter - .002". Though that gave a better average velocity, the SD was about 8fps. I sized another batch at groove diameter -.0005" to -.001", and the velocity was lower but the SD was only about 2fps.New velocities with the tighter slugs:7mm, 48" barrel, 2650psi1008fps - 106.3gr BBT 240fpe1028fps - 97.3gr Bowman 228fpeI'm thinking that the recent larger balance chamber volume could allow the valve to open with a higher regulated pressure. I tested charged to 3000psi and it seemed to do OK:7mm, 48" barrel, 3000psi1050fps - 106.3gr BBT 260fpe1090fps - 97.3gr Bowman 257fpeI put the 3000psi reg back in and will test it more thoroughly later this week, at the range.
Have you considered 257388 bullets for the .257? They are usually very accurate and you gain in BC and energy while still able to use 14" twist.
Quote from: rkr on December 22, 2022, 01:15:32 AMHave you considered 257388 bullets for the .257? They are usually very accurate and you gain in BC and energy while still able to use 14" twist.Yes. I have that mold and have tried them. Not quite as accurate as the 257420, but I might try them again.