Will make it short: don’t sell HPA conversions short.
Had to take a QB 5mm conversion apart for the first time in 18mo. due to a leaking valve body 0-ring. Has been under 850Psi pressure for all of that 18 months. When taken down, valve body was loose enough to move without unscrewing, which is NOT how I remember putting it together. Probably would have stopped leaking if I just tightened it again.
So long as I had it apart, though I’d do a little work to see if the shot count would be improved from it’s normal 45-50 shots @ 14.7 foot pounds (14.3 @ 680fps). At one point, had it running at 728fps (16.8 foot pounds) but with too few shots on a full tank of air.
Rifle had already had the basic mods. To try for a longer shot count, slightly lightened the valve stem return spring, decreased the striker spring strength, switched back to an standard bolt (lathed to fit the 5mm bore but no bolt probe), and lightened the striker weight.
(Most call it a “hammerâ€, but for me, if it moves in a straight line it’s a “strikerâ€, “hammers†move in an arc.)
(Old photo…but I’ve not had any reason to change anything, so no new pix)
click to enlarge
Vel. dropped from 680fps to 630fps (12.6 foot pounds). But the shot count more than DOUBLED.
Honestly, didn’t expect to go over 100 shots, which is why I had to tape on that last line of targets.
Vel. readings (rounded to the nearest whole number). Didn’t chronograph all 115 shots, but did take three strings of 10 shots at different tank pressure levels.
13CI HPA tank
3000PSI:
Shots #1-#10:
631
632
630
631
631
631
630
632
631
630
average 631fps +/- 2fps
2000PSI
Shots #55 -#65
632
630
629
631
631
628
629
630
631
628
average: 630fps +/- 4fps
1000PSI
shots #100- 110
628
626
626
627
627
631
626
625
626
624
average: 626fps +/- 7fps
Overall, of those chronographed shots at 3 different tank pressure levels, the highest vel. recorded was 632 and the lowest 624, so something like 8 fps variation over 110 shots.
May not care for the power trade off, but for my uses, its more than a fair trade.
