QB79 Ninja - The $200 PCP (PAL Required)

[rsterne]

This thread will be slightly different than most of mine, in that it is about building a very simple yet efficient PCP for about $200.... You won't need any special tools, and there are only two things to purchase:

An Industry Brand QB79 Airgun
A Ninja 13CI/3000 psi tank with an SHP Pro Regulator

What you end up with is a gun that looks like this.... It weighs 6 lb. 10 oz. and balances right on the foregrip....



and shoots like this.... In fact twice that many shots on a full tank....



The average velocity while above the regulator setpoint was 609 fps with Crosman 14.3 gr. CPHP pellets, which works out to 11.8 FPE.... On a 3000 psi fill, it works out to 135 shots and refilling at 1100 psi.... for an efficiency of 0.95 FPE/CI....

What does it take to accomplish this great combination?.... Would you believe mount the tank on the gun, fill it with air and start shooting?.... First a little history.... The QB79 is the "tanker" version of the QB78, which was developed from the Crosman 160, and manufactured in China.... It is available for between $100-$120, and is intended to use a CO2 Paintball tank, although many people substitute Paintball HPA setups instead.... So what did I do that was different?.... Instead of the commonly available 13 CI / 3000 psi tanks that have a regulated output of 800-850 psi, I used Ninja Paintballs new SHP Pro version.... The "SHP" stands for "super high pressure" (advertised at 1100 psi) and the "Pro" refers to their "360* twist technology" which allows the tank to be rotated so that the fill nipple and gauge don't interfere with the barrel.... I recently reviewed the SHP Pro, which lists for $80 US, here.... http://www.canadianairguns.com/showthread.php?1095

OK, so the QB79 is designed for CO2, but we're running 1100 psi, what about safety?.... What people forget is that CO2, while having a nominal pressure of 850 psi at 70*F, can produce pressures as high as 1900 psi at 120*F.... At 90*F, it's about 1200 psi, and at 100*F about 1400.... and we don't hear of QB79's exploding on a hot day.... About all that happens is that when it gets hot, the velocity drops, because the hammer spring peaks out at about 1100-1200 psi.... Having said that, there are a couple of recommendations I have on the QB79, whether on HPA or on CO2....

Don't remove the action from the stock with gas in the gun.... There are two additional stock screws that thread into the tank block and provide an additional safety measure should the other screws yield....

Don't remove the breech with gas in the gun.... The valve is only secured with one screw, but sits against a steel block.... That steel block is secured on the bottom by the stock locating stud and on top by the flathead screw in the breech, located under the barrel.... ALL THREE SCREWS must be in place and tight before the gun is pressurized....

The Ninja SHP Pro regulator is equipped with two burst discs, one to protect the tank from overfilling, and the other to prevent the output pressure from rising about 1800 psi.... This is ideal for our purposes, as those pressures can be reached in a CO2 system, and in fact CO2 tanks are rated at 1800 psi.... As long as you don't disturb the 1.8K downstream burst disc, your QB79 should be just as safe as running on CO2, IMO.... I chose the SHP because of its 1100 psi output (my tanks actually measured at 1200).... Remember I mentioned earlier that the hammer spring in a QB peaks at about that pressure?.... That means that the gun should be shooting pretty efficiently without having to change it.... If you are only running 800-850 psi the hammer strike is actually too strong, and the gun will be wasting air, and may experence hammer bounce.... It still might get lots of shots, but clipping a coil of two off the hammer spring would improve the shot count even further....

Please note, I made NO modifications to the gun in any way.... I did disassemble the breech and deburr it and the bolt and the rear section of the main tube, and I polished up the trigger and hammer and installed a lighter trigger spring and 2-stage plunger.... but those are all fine-tuning details that anyone might do to a CO2 version and have nothing to do with the performance, only functioning and shooting enjoyment.... You have to slide out the barrel to rotate the tank to install it, as the gauge and fill fitting won't clear.... but that gives you the opportunity to deburr the forward section of the breech so it doesn't keep shredding the O-rings anyway....

I think this conbination of the QB79 and the Nnija SHP Pro setup is just about perfect as an introduction to PCPs.... The price is certainly right, and the platform can be modified to your heart's content.... The target style AR2079 version isn't a lot more money, and the conversion process is identical.... IMO, the QB79 and the Ninja SHP Pro go together like bread and butter.... and just in case you might be interested in spicing up that combo a bit, stay tuned as I have a couple of simple mods in order as well....

Bob

[rsterne]

Today I removed the barrel and breech to ease the restrictions and increase the airflow.... I did NOT remove the valve from the gun, all mods were made to the barrel and bolt only, plus I installed a larger transfer port seal, from a Crosman Disco/22XX/13XX.... just the rubber seal only, not the metal sleeve.... I got that idea from Tim at Mac1, and it really works great, doing a perfect job of sealing the curved valve to the curved breech, and it's larger than you can go on the porting on a .22 cal barrel....

I drilled out the barrel port one number drill at a time, starting with the #28 (0.140") and ending with the #20 (0.161").... That makes the transfer port 75% of the bore diameter, which is as large as you dare go without creating loading problems for the pellet.... The area for the airflow is a third larger than stock.... I then drilled the nose of the flow-through bolt out one size at a time, starting from the #34 (0.111") and also ending at the #20, so that it was the same size as the barrel port.... That greatly enlarged the hole in the bottom of the bolt where the air flows through, and I used a 1/8" ball grinder tip in a Dremel to smooth and enlarge the slot formed until it was the same area as well.... After careful deburring of the inside of the barrel and bolt using a needle file, I reassembled the top end of the rifle and tested it again.... Here are the results....



The average velocity while on reg. jumped up to 738 fps (17.3 FPE), giving 48 shots starting at 2000 psi before the velocity dropped below 730, and a total of 58 shooting down to 1000 psi.... That works out to 17 psi per shot, and an efficiency of 1.10 FPE/CI.... Starting from 3000 psi, you should get 105 shots down to just under 1200 psi.... This mild mannered gun is beginning to show some real promise.... The Stage 1 mods to accomplish it can all be done with a set of numbered drills and a needle file, plus a rubber seal.... and without removing the valve from the gun....

For the next round, however, I will be pulling the valve out to improve the airflow into the valve, and provide more volume between the regulator and the valve to increase the average pressure available during the shot cycle.... I'm really looking forward to seeing the results....

Bob

[bitterman]

Very nice!!!!

Bruce

[rsterne]

Well, sometimes you pay for not keeping good notes.... I made an offset RVA, removed the rear cocking pin to convert to "cock on open", and machined a new downwards slot to hold the bolt back while loading the pellet....



The only problem was, that the version I previously did that on (in the photo above) had a shortened hammer and guide pin on the cocking block to allow the bolt to be drawn back further without the hammer hitting the cocking block when drawn back to the rear notch.... When I adjusted the RVA so that I could push the bolt down into the rear slot, the hammer spring didn't have near enough preload on it.... Rather than modify the hammer and guide pin, I made a spacer to slide over the guide pin on the cocking block to increase the hammer spring tension.... The spacer was 0.35" OD, 0.22" ID, and 0.30" long, so it adds that much preload, and allows the gun to function properly.... I then proceeded to test the velocity at various preloads, with the following results....



You can see the typical plateau in velocity you get on a regulated gun when the hammer strike is more than required.... The velocity doesn't increase, the gun just uses more air.... One slightly unusual thing on this graph, is that the efficiency falls off on the right side as well.... For some reason, the power drops off quicker than the air usage when the gun is detuned below 800 fps in this Stage 2 configuration.... My choice with this setup, at 1500 psi, would be to tune the gun for just over 900 fps (26 FPE).... I would expect about 50-60 shots at that power level.... Pushing for more power than that with a QB79, even with the "Maxi-Valve" simply wastes air....

Bob

[rsterne]

I finished and installed the stock today....



It's a Richard's Microfit Wildcat Thumbhome in Select Grade Black Walnut with Ebony Grip Cap.... beautiful figuring in the grain....



I had to shorten the forearm and inlet it for the tank block and regulator.... The finish is 7 coats of Watco Danish Oil, each coat wet sanded, ending with a white Scothbrite pad of 1200 grit equivalent and then a coat of Dark Minwax.... The scope is a Leapers 3-9 x 32 AO with Sunshade.... The gun weighs 8 lb. 7 oz. as shown.... The Air Stripper is from a Hatsan PCP, I threaded the barrel 1/2"-20 NF to accept it....

It's no longer a $200 PCP, however....

Bob

[rsterne]

Just out of curiosity I flung a few 18.1 gr. JSB Heavies through the Chrony.... They did 855 fps (29.4 FPE).... That's 3 FPE better than the best I ever did with a QB79 at 1500 psi with those pellets previously.... so the new "Maxi-Valve" certainly works great.... Also, so far, it doesn't seem to leak, despite not having the O-ring "squeezed" between the valve halves.... I would say the "Maxi-Valve" idea is a complete success in the QB79....

Bob

[rsterne]

Anything over 1/2" can be fitted, the Hatsan strippers are threaded 1/2"-20 NF.... You could probably even thread a 12mm LW barrel that size, the threads would just be very shallow.... The back end is located on an O-ring to prevent it from unthreading.... Not sure if it would work, may try it some day....

Bob

[rsterne]

Based on previous experience, where I got a significant gain on a 32 FPE QB78 running at 1600 psi, I cut off the flow-through bolt probe, squared off the end of the bolt flush with the back of the barrel port, drilled the bolt and installed a 3/32" diameter extended probe that is 0.10" longer than the original flow-through (because it pushes up inside the skirt).... Whereas on the previous time I did this mod I gained 32 fps, this time I lost 5 fps.... However, the ES seems slightly less (ie fewer high velocity shots).... The only explanation I have for this, is that on the more powerful, higher pressure version, where flow is more important, the thin probe is better.... However, on this version, running lower pressure and half the air volume through it, there is very little difference, and it isn't worth having to use a lathe to make the change to the bolt, simply drilling it out works just as well.... When I increase the power further, I would expect the thin, extended probe to work better, but of course I can't go back and test the flow-through now, all future testing will be with the 3/32" diameter probe....

Basically, either drilling out the flow-through nose.... or replacing it with an extended probe.... work equally well at this pressure (1200 psi) and power level (17 FPE)....

Bob

[rsterne]

Here are the results of a full string starting at 3000 psi using the Stage 2 "Maxi-Valve" setup running at a regulator pressure of 1500 psi.... I set the RVA to 2.5 turns out, which set the velocity just where the "knee" of the curve starts, with the 14.3 gr. Crosman CPHPs....



The average velocity was 908 fps (26.2 FPE) with a high of 914 and a low of 902 for an ES of 12 fps (1.3%) over the first 58 shots.... Shot 62 was just outside a 2% ES and wasn't counted, and the pressure at that point was 1300 psi.... As is typical when you tune a regulated gun to the "knee" of the velocity curve, you can shoot a couple of hundred psi below the setpoint before the velocity takes a dive.... The efficiency worked out to 1.07 FPE/CI, which for this power level in a QB79 is pretty astounding....

I'm extremely pleased with the results of this testing.... I think it would be very difficult to get more power than this from a QB79 without major changes to allow running a burst disc higher than the 1.8K which is required for safety the way the gun is produced.... While more power could be produced from a QB78 because of the larger tube, when you consider the simplicity of just screwing a tank onto a QB79 to convert it to a PCP it's a pretty impressive combination.... I will be re-fitting the 1200 psi standard SHP Pro regulator for final testing shortly....

Bob

[rsterne]

I made one more slight change today before the final testing.... The off center RVA screw was canting the cocking block slightly, and I noticed a slight scuff mark on the spring spacer.... I made a longer, one piece cocking block, with an integral spring guide and spring spacer.... The new cocking block is 1.25" long, of which 0.25" is a spring spacer that is 0.34" diameter, the other inch being full diameter to eliminate it canting in the tube.... That allowed me to shorten the RVA screw and just tidy things up a bit.... It shouldn't have changed the performance at all, but in fact I picked up about 10 fps, and gained a bit of efficiency as well.... The Preload numbers have changed, as I can't put quite as much preload on the spring at maximum with the shorter spacer.... Here is a new graph of Velocity and Efficiency vs. Preload using the JSB 15.9 gr. Exact Jumbos pellets....



As per my usual practice, I selected a preload on the knee of the curve, just below the plateau, in this case 4.5 turns out.... I refilled the tank to 3000 psi and shot a string with the following results....



I got 80 shots between 844 and 856 fps, an ES of just 12 fps (1.4%).... and was able to shoot down to 1100 psi before the velocity dropped below 840.... The average was 849 fps (25.5 FPE) at an efficiency of 1.21 FPE/CI.... During shooting the string, I shot 5 shot groups and sighted in the scope in my 20ft. indoor range.... By the time I was done, it was punching group after group in the same place on the target that were only 1/4" across, barely bigger than the pellet.... I can't wait to try this rifle out at longer ranges.... This is just about the perfect velocity to shoot the 15.9 gr. Exact at, putting it right in the middle of it's sweet spot for the Ballistics Coefficient....

This has been an exciting project, and I'm extremely pleased with the final outcome....

Bob