I got my FD-PCP in the mail today, filled it to 1500 psi and shot a few strings with JSB 14.3 gr. Express.... I had a few teething troubles, the first of which I never did fully figure out.... The first string was great, high 800s, about 9 shots within 4% ES ending at 1500 psi.... I tried another string, didn't change anything, and the string started in the high 700s, with the first 6 shots way to slow to include in the string.... The next string was even slower, so I figured something had to be changing.... I knew the trigger could alter the velocity and when I checked it, it wasn't fully back (and the screws not that tight, it may have slipped forward), so I reset it fully aft, and while I had it off did a quick trigger tune, as the stock setting was horrible, the sear engagement was wayyyyyyyyyy too long and therefore creepy.... I shortened up the sear engagement to about 1/16", clipped 2 coils off the trigger spring (the adjusting screw was nearly falling out), and it was much improved.... With a bit of polishing it should be every bit as good as the QB trigger it is based on.... Incidently, this trigger uses the spring-loaded 2-piece blade like the ARs, so it has a 2-stage feel to it....
Anyway, with the trigger fully back, the gun wouldn't cock, the bolt slot wasn't quite long enough, so I had to move the trigger ahead about 0.020" and then it was fine.... The next string duplicated the first, so I thought I had found the problem, but the next string was once again slow.... and now I knew nothing was moving.... I found that the set screw in the bottom of the striker was loose, as was the slotted screw under it, and I tightened those and proceeded to find out what changing the preload would do.... I was curious if there was enough adjustment to get to coil bind, so I removed the lock nut and washer from the preload adjusting screw and wound the screw in until it was flush with the threaded plate, and the gun would still cock, no coil bind.... I filled to 1500 and shot a couple of shots, just over 940 fps which is 28 FPE.... Pretty impressive performance, I have to say, this thing must breathe pretty well.... I then started backing out the preload and recording the velocity, filling to 1500 each time, and the gun was solidly on a plateau, I had to back the preload out 5 turns before it even started to loose velocity.... I can only imaging what an air hog it would have been with the screw cranked in flush.... Anyway, at 6 turns from flush with the plate (which I took to be the maximum usable preload), I reinstalled the washer and lock nut, and the screw was flush with the surface of the nut when the slot was vertical, and the velocity was just beginning to drop, so I used that as a starting point for further preload testing.... As I backed off the adjusting screw so that it protruded from the lock nut, the velocity dropped rapidly, and at 3 turns out it was under 500 fps.... This is with the trigger as far back as I could get it and still cock the gun.... I graphed the results, and then proceeded to shoot some strings.... Here are the graphs.... NOTE: PELLET WAS 14.3 GRAIN, NOT 15.9 GRAIN !
With the screw flush with the surface of the lock nut the gun was shooting right at the top of the sweet spot, but only got 6 shots before the velocity dropped more than 4% below the peak.... So, for my gun at least, there is no point in going for any more preload at 1500 psi.... At 1 turn out, I got a beautiful string of 9 shots, starting right at 1500 psi and ending at about 1100.... The efficiency was very good at 1.15 FPE/CI, and the string started out fairly quiet and by the end it was really loud.... At 2 turns out from flush with the locknut, the first shot was only 646 fps and really quiet, and the velocity gradually built until I could start counting shots at about 1350 psi, and the string ended at 1000 psi.... In other words, if you are using the recommended 1500 psi fill, all the usuable adjustment range (at least on my gun), with the trigger set back as far as you can and have the gun cock, occurs from where the adjusting screws is flush with the lock nut to about 2 turns out from there.... Since the adjustment is so sensitive, and I'm sure there are differences between hammer springs and other parts (not to mention trigger position, which makes a huge difference) the only way to determine this for your gun is to basically duplicate my testing method.... Keep cranking in the preload until the velocity plateaus, and then you know how far is too far, back it off until the velocity starts to drop, and then a bit further until you find your best tune....
My initial conclusions are that this gun is a powerhouse; my best string of 9 shots, from 1500 psi down to 1100, averaged 24.7 FPE at an efficiency of 1.15 FPE/CI.... I think the biggest challenge is going to be figuring out how to get a longer shot string within a 4% ES.... I'd like to set this gun up as a Grouse gun for my son, as a repeater using the MRod mags, and the ideal would be 20 shots (2 magazines) at 20 FPE with 18.1 gr. JSB pellets, which I know to be a deadly combination on Ditch Chickens.... I have a feeling that to accomplish that, I will have to restrict the porting somewhat to flatten out the shot string and increase the usuable pressure range beyond its current 400 psi....
Bob
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