New QB 78 on HPA - (Now .25 cal)

[rsterne]

I finally got the .25 cal version assembled today, and did the initial testing.... I filled the tank, recorded the fill pressure, shot 10 shots, and recorded the ending pressure.... for every hammer spring preload setting from maximum to 7 turns out.... The regulator setpoint on the 13 CI 3000 psi tank is 1600 psi, and the gun is equipped with a 1.8K burst disc for safety.... I was using 25.4 gr. JSB Kings, and here are the results....



You will note that the last 3 turns of preload on the hammer spring do nothing to the velocity, but as usual the air consumption keeps increasing.... That is because the valve is closing too late to do any useful work, so the gun is just wasting air.... Here is the data for the shoulder portion of the curve in table form.... The number of shots are extrapolated from the pressure loss over 10 shots, so may not be exact....



I added a column for the "Bore Ratio", which is the barrel volume divided by the volume of HPA released during the shot.... You can see that once the pellet has travelled half the barrel length (Bore Ratio = 2.0) at the point the valve closes, adding extra air does virtually nothing (except waste air).... The more I experiment with regulated guns, the more I find this to be the case.... If you tune for where the velocity just starts to drop, you can often get the same power and use about half the air that you would running a "dump shot"....

I also tested other pellets with the hammer spring set at maximum, just to get a feel for what the gun can do.... Here is the data....



I'm pretty pleased that I got nearly 60 FPE from a QB 78 running on only 1600 psi.... I will be doing a bit of playing to get the final tune, which will probably be about 4 turns out.... I hope to get about 30 shots at 45 FPE with the JSB Kings.... The only work left to do after that is blue the barrel, stick a scope on it, and sight it in....

Bob

[rsterne]

I topped up my SCUBA tank with my ShoeBox this afternoon, and while it was ticking away I blued the barrel and touched up the bluing on the breech and bolt.... Then after dinner I reassembled the gun, set the RVA at 3.5 turns out, filled the tank to 3000 psi and shot a full string.... Here is a graph of the velocity, using 25.4 gr. JSB Kings....



To say I was pleased would be an understatement.... The string ended at 1300 psi, and only the last shot was low, at 907 fps.... I had 32 shots between 915 and 923 fps, an ES of only 8 fps (0.9%).... I was able to get more shots than I was hoping for at that power setting, which worked out to nearly 48 FPE.... That is excellent power from a regulated pressure of just 1600 psi.... Here is a summary of the results....



As you can see, the efficiency was a bit better than I reported previously, at 1.03 FPE/CI.... The gun is fitted with a Bushnell 4-12 x 40 AO scope and weighs 8 lb. 2 oz all up.... Here is a photo of the finished product....



The 13 CI 3000 psi tank acts as the forestock, and the gun balances just in front of the stock securing screw, which I find very nice.... The tank block incorporates a Picatinny rail for mounting a BiPod or other accessories.... I'm very pleased with the outcome of this project and I can't wait to see how accurate the Lothar Walther barrel is.... I am missing the safety lever, so if anyone has a spare, please let me know....

Bob

[bradyman1]

Great job? Those are fantastic numbers.

[rsterne]

I took the .25 cal QB out today and sighted it in and tried five different pellets.... I tested the 25 gr. JSB Kings, the 26 gr. Polymag Predators, 28 gr. Benjamin Domes, 28 gr. H&N Hunter Extremes, and 31 gr. H&N Baracudas....  At 35 yards you could pretty much cover every group with a quarter, and some were dime sized.... The tightest group went to the new H&N Hunter Extreme, which made me sit up and take notice.... five shots easily covered by a dime.... The Baracudas had 4 nice and tight with one flyer.... 

I moved out to 50 yards, and the Benjamins were starting to open up a bit.... and there was still a flyer with the Baracudas.... The other three were hanging in at just over the size of a quarter.... At 75 yards, there was a definite trend with the Hunter Extremes, the groups were starting to detereorate with distance, it was actually the worst group of the five pellets, at about 1.75".... There were now only two pellets really worth checking further out, the JSB Kings and the Predators.... both hanging in at around 1" C-T-C....

I set up 3 targets at 100 yards, and the JSBs and the Preds were both shooting about a 2" group, but the only way I could get them on paper was to back the scope down to 4X and use the post in the Multi-X reticle on the top of the target.... I'm sure that with a MilDot reticle I could have done better.... The interesting thing is that the H&N Hunter Extremes, which had the best group at 35 yards, had now opened up to nearly 5" across.... This is the first time I have used these pellets, and you can be sure I'll be keeping track of what they do in other guns....

Bob

[TimmyMac1]

You'll notice the Barachudas have some undersize ones in there and they are Not Match designation. They are Standard veriety. A 31 grainer graded pellet will do better. I don't think anyone has those but if they do I'd like to get some.
The Heavy hollow points have a basic instability issue as they seem to degrade in BC the further you reach. They are not bad at 35 out of most stuff and can be spectacilar inside of that range but not so good when you reach way out as the things slow to quickly and fly a lot sidewaysish. In other words the hollow wants to turn them around and the skirt turns them back so they are going down range swapping sides and flying sideways enough to ruin any chance of decent retained energy. The upside is they hit stuff and turn which can end up being a 4 foot circle of feathers. Quite satisfying.
Kings are hard to fault in most .25 cal guns.

[rsterne]

The Kings and Preds both shot well.... but the POI at 100 yards on the Preds was 3-5" lower (I didn't measure it, going by eye).... and the Hunter Extremes were in between.... Since they all shot to about the same POI at 50 yards and less, that gives an indication of the relative BCs.... For a combination of accuracy and retained energy at long range, the Kings are still king, IMO.... Sorry, had to say it....

Bob