Impact JSB 33.95 FPS

I shoot .30 cal with a 700mm barrel, but I am pretty certain he should be getting much higher than that, Have him check the C3 bumper. If it has a DURO90 o-ring, it may need changing. Be aware that damage is not always evident at a simple glance. I change them out for Delrin C3 bumpers. The only reason I wouldn't is if I needed to shoot very low power and caliber.

Sounds like the C3 is shot. Change it out for delrin. I’ve had one impact go maybe a couple tins of pellets and another last thousand or so before C3 was toast. Swapped out for delrin and never look back plus gain a few more feet per second. 


John T

I do not believe that is normal, but you have too much spring tension that will reduce the FPS . Whenever I adjust my spring tension using a chrony I find a speed whatever it my be it should be a very consistent FPS I find a high reading but a consistent reading. Sometimes I have to loosen spring tension to find consistent FPS readings. I hope that helps good luck.

810 from a impact max power is normal. My stock 25 impact with 600mm pellet liner max out to 830 fps with the jsb heavy mk2. But if you changed the c3 o ring to delrin. You will get to about 860 fps with the same setting. And if you enlarge the transfer port and probe, you can get maximum up to 900 fps with the mk2 heavy. 

While he’s changing the C3 bumper, might was well add hammer weight as well. The 25’s and 30’s seem to need the extra weight to push extra weight.

Should be like 930