A New Method for Increasing the Efficiency of a PCP

[rsterne]

Yesterday and today I worked on fitting an SSG to my QB Ninja.... The gun is regulated at 1200 psi, and is modified to cock-on-open.... The best previous tune delivered 81 shots at 850 fps from a 13 CI tank filled to 3000 psi and refilled at 1100 psi.... For the preloaded spring for my "Stopped Spring Guide" modification I used two 1.75" x 0.040" wire springs on a single 7/32" guide, with 10-32 nuts both ends (the front one turned to fit inside the hammer).... The adjusting bolt is a 3/8" - 24 NF bolt drilled for the guide, and it is carried in a short piece of aluminum which is bolted inside the QB tube.... Here is the previous cock-on-open RVA, and the new SSG setup, positioned relative to each other the way they sit inside the tube....



In the uncocked position, you will notice that the end of the guide is basically flush with the end of the tube, but of course it sticks out when cocked.... I cured that later, as you will see below.... There are two screws to mount the aluminum block in the tube, a short one on the bottom and a long one on top, that goes through the breech and holds it down.... Since I didn't need the back part of the bolt that normally moves the pin for the cocking block, I cut it off flush with the back of the setscrew that holds the bolt handle, and the hole for the top screw for the SSG is immediately behind the bolt when fully back.... You can see the relative positions of the parts in the photo below....



In order to cover the back of the breech, I made a special bolt from a short metric flat-head screw by turning the head to 1/4" OD and thinning it slightly.... and then milled a recess in the breech cap to allow it to sit below flush, as below....



The bottom SSG mounting screw interfered with the trigger, so I used a low profile SHCS and counterbored a recess in the trigger as shown to miss it....



Since when cocked the guide and nuts stick out the back of the tube, and I needed something to mount the back of the trigger into, I made a cover from aluminum as shown below.... It is tapped for a 10-32 screw to attach the back of the trigger through the original hole in the back of the tube.... It does double duty to prevent the gap adjusting bolt from rotating as well.... The lip that sticks forward inside the tube has a flat milled on the top that just clears the flat on the bolt, so that once inserted the bolt can no longer rotate.... You do the adjustment with a socket wrench and then slide the cap in and bolt the trigger in place and the adjuster can no longer rotate....



When the gun is reassembled, the only change in appearance is the extended aluminum cap on the back of the tube, and the head of the upper screw on top of the breech under the scope.... It doesn't interfere with my scope rings, although something to consider if yours were in a different place....



During final tuning today I found a few more fps, and the gun now averages 851 fps (25.6 FPE) with the SSG one turn out from touching the hammer (0.042").... The ES is narrower than before (it was 12), only 8 fps over the first 86 shots of a 90 shot string down to 1100 psi.... Shot 91 was 838 fps.... This SSG gave me a gain of 9 shots over the previous arrangement (11%), increasing the efficiency to a pretty stunning 1.36 FPE/CI.... Here is the shot string....



I am now a firm believer in the SSG system as having the potential to increase the efficiency of a PCP, even a regulated one that was already pretty good.... If you can hear the telltale BUURRRPPPP of hammer bounce in you gun, and getting rid of it has been a problem for you, considering this modification could be the way to go....

Bob

[rsterne]

For some time I have been wondering how effective the SSG would be on a CO2 gun like the Crosman 2260.... I had the opportunity to try that out today.... The testing started out with the gun configured exactly as a stock 2260.... Stock valve, hammer and spring, stock transfer and barrel ports, and a 24" barrel, although the gun did have a Crosman steel breech on it.... I loaded a brand new 12 gr. CO2 cartridge, fired the gun twice to pierce the cartridge, and then shot a string using 18.1 gr. JSB Heavy pellets until the velocity dropped to 300 fps, at which point I stopped.... I fired one shot every 30 seconds, and the temperature was 68*F.... I then repeated the testing using a much lightened hammer that was 50% of the weight of a stock one.... In both cases the stock rear cap was in place, so the preload on the 2260 spring was completely stock.... I then fitted an SSG, using the same spring, with as much preload I could use and still cock the gun with no gap between the end of the guide and the hammer.... I adjusted the gap to 0.020", installed a new cartridge, fired two shots to pierce it, and then fired another string.... The results are shown in the graph below....



There are some interesting things to note.... In stock form, even though I was only shooting every 30 seconds to limit cooling, the velocity declined in a near linear fashion for 30 shots, and then dropped rapidly, with a total of 37 shots above 300 fps.... I calculated the FPE of every shot and added them up, getting a total of 341 FPE from a 12 gr. cartridge.... With the lighter hammer, the initial velocity was lower, but within 15 shots it leveled off, staying within a very narrow ES for another 30 shots before dropping, with a total of 53 shots above 300 fps.... The total FPE from that cartridge was 457 FPE, which is pretty good, and a 34% improvement over stock.... With the light hammer and the SSG, the velocity stabilized quite quickly, and I got over 60 shots before it started to decline.... and the initial decline was not as dramatic.... and I got a total of 78 shots above 30 fps.... over twice as many as stock.... The total FPE from a cartridge with the SSG fitted was 627 FPE, breaking my own previous record of 608 FPE with an 18" barreled 1750.... That represents 84% more energy from a 12 gr. than I got stock....

I am extremely pleased with the huge increase in FPE per 12 gr. CO2 obtained by using the SSG.... This was, additionally, the first attempt using an SSG on a CO2 gun for me, and breaking the 600 FPE mark without tuning was pretty impressive.... I look forward to trying some different settings to see if I can get even more energy from a 12 gr. in the future.... but right off the bat, we know it works great on CO2....

Bob