A New Method for Increasing the Efficiency of a PCP

[rsterne]

I tried this idea on a regulated PCP today, and it went very well.... The gun was my QB79 Ninja, regulated at 1200 psi, that shoots 25 FPE with 15.9 gr. JSB Exacts.... I used two 1.75" x 0.040" springs on a 7/32" guide, giving a total spring length of 3.4" and a rate of about 7 lb/in.... I tried three different amounts of preload, 0.70", 0.80", and 0.90", and adjusted the position of the end of the guide from 1 turn of compression against the hammer to 3 turns of gap (24 TPI), so the total range of adjustment in position was 0.167".... Here is what happened....



You can see that the maximum velocity this gun can reach is ~850 fps with these pellets.... As I increased the preload, the velocity at any given gap between the end of the guide and hammer increased, as expected.... For instance, with 2 turns of gap (0.084"), the velocity with 0.70" of preload in the spring was 760 fps, with 0.80" of preload it was 790 fps, and with 0.90" of preload it was 820 fps.... With the guide just touching the inside of the hammer, the velocities were 836, 840, and 845 fps.... and with the guide pushing on the hammer (actual preload on the hammer at rest), the velocity was within 2 fps with all preloads, about 847 fps....

The interesting part of the graph above, is the efficiency curves (the dotted lines).... With the guide pushing on the hammer, the efficiency was horrible, about 0.55 FPE/CI.... I think what is happening is that the large amount of preload is slowing the closing of the valve, and causing what amounts to a dump shot.... Once the guide is clear of the hammer, however, the efficiency is greatly improved.... With it just touching, it is 1.15 FPE/CI, which is about the same as I used to get on the knee of the curve with the original setup.... As soon as there is a gap, and it doesn't need to be much, just 1 turn (0.042"), the efficiency is great, between 1.27 - 1.32 FPE/CI at all preloads.... The efficiency at any given velocity is slightly better with more preload, which to get that same velocity requires more gap between the hammer and the guide....

I ended up using the 0.90" preload and 1/2 turn of gap (just 0.020") between the end of the guide and the inside of the hammer.... This gave me 843 fps (25.1 FPE) at an efficiency of 1.27 FPE/CI, which is better than I had with the original setup.... The ES, at 10 fps over a 20 shot string, is a bit better, and the SD is definitely better than before, with by far the majority of shots being within about 4-5 fps, with only the occasional high or low shot.... possibly due to differences in individual pellet weight or fit.... All in all, I am very pleased with the results.... It would appear that this setup does, indeed eliminate hammer bounce.... I think the efficiency graph, being so flat once there is a gap, proves that point....


Bob

[rsterne]

I pulled apart my 2560 today and swapped it over to the new style one with the internal stop.... Here are the old and new parts....



I am now using a 10-32 machine screw with a piece of 7/32" OD tubing slid onto it for the guide.... Then you slide on the spring and an aluminum collar that hits the end of the gap adjusting bolt when cocking the gun and compresses the spring.... The stop nut is turned from a piece of 5/16" rod, drilled and tapped 10-32 and tightened against the end of the brass tube.... I start out a bit longer than I need and keep shortening the tube until I can get the gun shooting on the plateau with no gap.... Once I get the right length, I install the stop nut with a drop of blue Loctite, tightened against the end of the tube.... The stop nut has a slot in the end for this purpose.... The gap adjusting nut is drilled just over 5/16" because the stop nut has to slide through it.... The end cap is tapped 7/16"-20NF for the adjusting bolt, and then counterbored just over 7/16" to miss the cocking collar.... At the bottom of the photo is the original SSG I made.... You can see how much shorter the new guide is.... Below is the new SSG assembly, ready for installation....



When the gun is uncocked, the stop nut is recessed about 1/2" inside the gap adjusting bolt, so you can't see it.... Compare this with the one above, where the stop nut and bumper O-ring (no longer required) are outside the gap adjusting bolt.... Below is what the gun now looks like when cocked....



This installation is much cleaner, and there is no way you can pinch your fingers like could happen with the old one.... However, you still have a cocking indicator because the stop nut (the part with the slot) is visible when cocked.... With no gap I had about 985 fps, which I could adjust down to whatever I wanted.... At 1-1/2 turns out the velocity was just over 900 fps and the gun was very quiet.... I ended up about 1 turn out, with a average velocity of 960 fps, the same as I had before.... I might have lost a couple of shots compared to the first SSG, but I have still gained about 40% more shots per fill than without the SSG.... and I no longer have to worry about pinching my fingers and I can wrap my thumb over the wrist of the stock again because is it so much more compact.... The efficiency is around 1.40 FPE/CI....

Bob