Adjustable piston for Stoeger XP4/Beeman P17

[happymecanic]

Recently bought a Stoeger XP4 ssp pistol, which is almost the same as the well known Beeman P17. The main differences are the plastic upper and lower frames, and the piston. The piston itself has a shorter body, and more ''dead space'' than the P17. Actually, there was close to 1/8'' of ''dead space''. That is causing more side-load when cocking the pistol, and poorer performance than a stock P17.

So I made a longer (same length as the P17), adjustable brass/steel piston to replace the short, aluminum stock one. It's adjusted to just ''kiss'' the valve wall, much in the same way a flat-top piston is adjusted in a 13xx pistol. The velocity gains were substantials: it started at 340-345 fps with JSB Exact RS 7.33 grains with the stock piston, and went to 420-425 fps with the adjustable piston, using the same pellets. That's a 80 fps gain, quite significant! The new piston head barely clears the air inlet hole, but it clears sufficiently to let air in efficiently. It also added some heft to the pistol, which I find help balancing and stabilize the pistol. Cocking is also much smoother, no more side load/friction felt. Cocking force has increased noticeably, but it's not worse than a stock, healthy P17. All in all, very satisfied with this mod!

[bandg]

Very nice work.