I'm currently dealing with a similar problem at the other end of the spectrum. I'm working on a 12 FPE .177 and trying to bring in lots of shots with a low total spread. So far, I can get 60 shots with a total spread of 40 fps on 700 psi of air. Filling to 1700 shooting to 1000. FPS running 800 to 840, but for 12 FPE, 820 is max speed with CPL.
First thing was adding a McMaster spring, and replacing the spring guide with a delrin guide. Then I went off the charts and made an over-sized valve stem at .495. It valve locked at 1500 psi and shot in the high 700s. I cut the valve stem down to .470 and put in a 2.5mm port and that's where I'm currently at. I'm going to do some more work on it to make the firing sequence more consistent, because it just randomly tosses 11 fps spikes in velocity. If I can get the spikes flattened, part of the extreme spread will be gone and that will put me closer to my desired goal. I want to get it down to 20 fps spread over the 60 shots.
If you decide to trim down your valve stem, go very slowly. It generates power faster than you may think. The air flow increases and is harder to bring back under control. The first time that I played with a reduced valve stem, it was like having the factory spring back in. The the shot string was a downhill drop. No curve at all.
JK's Airgun Forums, moderators, and administrators are not responsible for any problems that may occur from reading or using content posted on this forum, as they are the exclusive responsibility of two parties: the person who posted it and the person who acted on said posted information.Use of our forums by people under 18 years old is allowed only with legal guardian(s) present..