Air Gun Home :: View topic - QB79 Ninja

rsterne · Moderator

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://airgunhome.com/agforum/viewtopic.php?t=9426

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

Additional information, March 2015....

Recently one of our GTA members, Ribbonstone, noticed a slight bulging of the end of the tube on his QB79, in the thin area adjacent to the screw holes for securing the tank block.... The gun was operated for a time at 1250 psi.... In response, I have done an analysis of the tank block attachment, which is detailed in this thread....

http://www.gatewaytoairguns.org/GTA/index.php?topic=105712.

If you are building a QB79 HPA conversion of any kind, you should read that thread.... If you are using more pressure than the 1100 psi factory setting for a Ninja SHP regulator, I am now recommending that you pin the tank block with two additional 4mm or 8-32 screws, or a 5/32" shear pin, between the O-rings.... If you want to be more conservative, you might want to pin the block when using the SHP regulator.... or even with a standard 850 psi Paintball regulator.... Complete information is in that thread....

Bob

rsterne · Moderator

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

rsterne · Moderator

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 · Moderator

Here is a photo of the valve I will be installing.... The 1/4" wide aluminum ring is all that is left of the front part....

It was epoxied onto the rear portion using a #113 O-ring for a spacer (just touching).... When the JB Weld was 4 hours old (not sticky but not hard) I cut the O-ring and peeled it out of the groove and then cleaned up the front of the groove with a razor blade to remove any excess epoxy.... Once hardened, I will turn it around and face off and bore out the inside to increase flow into the poppet seat....

Bob

rsterne · Moderator

I did some more work on the Stage 2 "Maxi-Valve" today.... The concept is to turn all the volume between the valve seat and the QB79 tank block into usuable valve volume.... Here are the results....

I machined a 0.10" long spring seat into the end of the poppet to fit the stock QB valve spring and then tapered the sides of the poppet on a 5* angle to reduce the OD to that of the spring at the seat.... The support for the front end of the spring is made from a 1.5" long 8-32 SHCS with the head turned down a bit to fit inside the QB spring.... I made a tapered brass collar to slide over the screw to form the spring seat, and then drilled out a piece of 3/16" brass K&S hobby tubing to fit over the screw as a spacer to set the height.... The tank block will be drilled and tapped on center for the 8-32 screw....

It turns out that in a stock QB valve, the installed spring is compressed to 1/2" long, so as long as I have that distance between the spring seats I will have stock preload.... It works out in my QB79 that I need the front spring seat 1.035" from the tank block to acccomplish that.... Here is a view of the inside of the valve, showing what the flow is like....

If you compare that to a stock QB valve, you will see a huge difference.... The ID of a stock valve is 7/16", my valve is bored out to 1/2", which doubles the clearance around the poppet.... The aluminum ring I glued on is 1/4" long measured from the O-ring groove.... After the epoxy hardened, I machined out the front of the valve on a 30* angle until there was only about a 0.020" lip remaining.... That took the taper about half way to the seat.... I then changed the angle to 15* and knocked off the corner to smooth the flow even more....

The throat of a stock QB valve is 0.198" and the stem is 0.116".... I drilled the throat out to 7/32" (0.219") so that the throat area is larger than the rest of the porting I did in Stage 1.... I also blended the bottom of the exhaust port into the throat using a 5/32" ball grinder in a Dremel.... The distance from the valve seat to the tank block is just over 2", which gives a basic volume of 14.5 cc.... but by the time you subtract the volume of the poppet, spring, and forward spring mount, I ended up with a whisker over 12 cc.... While it's not as much as I'd like, it shouldn't cause too much loss in efficiency until the power goes over 25 FPE.... I'd frankly be very surprised to see that power level, even with my "Maxi-Valve" installed, when running at only 1200 psi....

Bob

rsterne · Moderator

I drilled and tapped the tank block for the front spring mount and assembled the Stage 2 version of the gun this morning.... I was quite pleased with the results, as I have exceeded stock Disco performance with this version.... Here are the results from a 2000 psi to 1000 psi (tank pressure) string....

The first 22 shots averaged 864 fps (23.7 FPE), dropping below 860 fps only once the pressure dropped below 1200 psi which is the setpoint on my Ninja SHP Pro regulated tank.... That should give 50 shots on a full tank, which is pretty decent at that power level, even though the efficiency (to me) is a bit disappointing at only 0.74 FPE/CI.... It is quite possible that reducing the hammer spring preload would increase this efficiency somewhat.... The gun may well be operating on the "plateau" of the velocity curve, and reducing the hammer strike a bit may well not drop the velocity while increasing the shot count.... I'm pretty sure that dropping the velocity just a bit (20-30 fps) could add maybe 10-20 shots to the string, so that is something worth experimenting with at a future date.... If we could get the efficiency up to 1.0 FPE/CI at 22 FPE, 73 shots should be possible....

With the relatively small 12 cc plenum, which is only 1/2 cc per FPE at this power, the average pressure available per shot is less than 900 psi.... How is this possible?.... We start out with 12 cc (0.73 CI) of air at 1200 psi (83 bar), which is 60.6 CI of air a 1 bar.... Each shot is using 36 psi from the 13 CI tank, which is 32.3 CI of air at 1 bar.... Assuming the regulator response time is too short to add any useful quantity of air during the 1-2 milliseconds before the valve closes, at that moment we only have 60.6 - 32.3 = 28.3 CI of air at 1 bar left in the valve.... That means the pressure has dropped from 1200 psi to (28.3 / 60.6) x 1200 = 560 psi, so the average pressure during the shot is only 880 psi.... It shows how important plenum volume is in a regulated gun.... If we had double the plenum volume, the average pressure would be 1040 psi, much closer to the 1200 psi setpoint.... and the efficiency would be significantly higher.... On the other hand, this is pretty remarkable performance on a QB79 from only 1200 psi, thanks to the much increased volume of the "Maxi-Valve"....

About the only thing left to do is to test this gun running at 1500 psi, which is an easy level to achieve with the Ninja SHP Pro regulator and still be within the safety levels provided by the 1.8 K burst disc.... How to change the regulator to that output is in the separate thread on the Regulator referenced in the first post in this thread.... As time permits, I will be doing that and reporting the results....

Bob

rsterne · Moderator

I swapped out the tank for the other one that I had set up for 1500 psi.... I wasn't quite sure what to expect, as normally that is too much pressure for stock hammer spring preload on a QB78.... As it turns out, with the smaller plenum of the QB79, the pressure drop during the shot compensates for the extra pressure at the beginning of the shot.... allowing the stock hammer spring to work OK.... Here are the results of the Stage 2 tune running at 1500 psi....

As before, the starting pressure was 2000 psi, and the ending pressure was 1000.... I got 16 shots averaging 918 fps (26.8 FPE) down to 1400 psi before the velocity started to drop below 900 fps.... The fact that the drop in velocity starts 100 psi below the regulator setpoint indicates that this tune is operating very close to the "knee" of the curve where I like to tune my regulated guns.... The efficiency was higher than at 1200 psi, working out to 0.80 FPE/CI, and I should be able to get 42 shots @ 27 FPE on a 3000 psi fill.... That a pretty serious hunting platform....

When I did the calculations for the pressure during the shot for the 12 cc Plenum, I get 836 psi at the end of the shot, and an average pressure of 1168 psi during the shot.... That gives a pretty good indication of why the stock hammer spring preload is working so well with this setup.... It also confirms that running 1200 psi on the Stage 2 tune is running less efficiently than it could because the hammer energy is too much for the 880 psi average pressure.... I now have no doubt that reducing the preload a bit on that tune will increase the shot count with little or no velocity loss.... I'm going to have to give some thought to what is the easiest way to accomplish that, as fitting an RVA to a QB isn't the easiest thing to do, and I want it adjustable, I don't want to just clip the hammer spring....

Bob

rsterne · Moderator

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 · Moderator

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 · Moderator

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 · Moderator

Today I refitted the stock Ninja SHP Pro regulated tank to the Stage 2 QB79.... I then shot 10-shot strings to check the velocity and air pressure drop for different hammer spring preloads so that I could calculate the efficiency.... Here are the results....

As I suspected during the initial testing of this version, the stock hammer strike was too much, wasting a significant amount of air.... The original tune suggested about 50 shots on a fill, but if the efficiency could be bumped up to 1.0 FPE/CI I predicted 73 shots.... Now I'm pretty confident that can be met or exceeded, since at 4 turns out I was seeing an efficiency of about 1.15 FPE/CI.... Now when using only 10 shots, and a gauge with 100 psi increments, it's easy to have some errors creep in.... but that 10 shot string only used 220 psi, or 22 psi per shot.... while returning 845 fps (22.7 FPE).... I'm looking forward to running a full string starting from 3000 psi as soon as I have my SCUBA tank topped up....

Bob

rsterne · Moderator

Here is the shot string with the hammer spring preload at 4 turns out.... This is for the Stage 2 tune with the "Maxi-Valve", using a stock Ninja SHP Pro regulated 13 CI tank filled to 3000 psi.... I was using 14.3 gr Crosman CPHP pellets....

The average velocity was 847 fps (22.8 FPE), and over the first 80 shots the highest velocity was 854 fps and the lowest 841 fps, for an ES of 13 fps (1.5%).... Only the last shot dropped below 840, and it still fell within 2% of the highest, and the pressure at that point was 1070 psi.... That works out to an efficiency of 1.07 FPE/CI, interestingly the same figure as I got yesterday shooting at 1500 psi....

Getting 80 shots in a flat string at slightly better than stock Disco power is nothing short of amazing.... I think this is the way I'm going to leave this rifle.... It makes one heck of an entry level PCP.... Here is a summary of the three tunes using the Ninja SHP Pro Regulated 13 CI tank, shooting 14.3 gr. Crosman CPHPs....

Stock QB79: 135 shots at 609 fps (11.8 FPE)
Stage 1 Tune (top end work only): 105 shots at 738 fps (17.3 FPE)
Stage 2 Tune (Maxi-Valve): 80 shots at 847 fps (22.8 FPE)

Note that in order to get the extended shot count in Stage 2, I had to REDUCE the hammer spring preload from stock.... I have a feeling that the shot count in a Stage 1 tuned gun (and possibly even a Stock gun) could be improved a bit by the same method.... Although I chose to use and RVA and convert the gun to "cock on open", simply clipping a bit off the hammer spring would have the same result....

Although my Maxi-Valve, IMO, is getting just about all the performance possible from a QB79 at this pressure.... I would like to point out that there are commercially available valves from several sources that have the piercing pin and filters removed and the sides of the valve opened up to increased flow into the valve.... I'm guessing that they would be in the 800+ fps range when used with the porting done in Stage 1, when operating with this tank and reg.... I would highly recommend the Ninja SHP Pro 13CI Tank and Regulator combination as an inexpensive and yet excellent way to convert a QB79 to HPA.... All the same mods that are used in the CO2 versions also work to enhance the performance of the PCP version, which is a big bonus....

Bob

rsterne · Moderator

Sometimes leaks can be a good thing.... I've had a slow leak in this gun virtually since new.... It lost about 600 psi the first day, then 3-400, then a trickle.... I tested everything with soapy water and couldn't find anything, which pretty much meant it was the valve.... I pulled the breech, and found a very small leak coming out the exhaust port, so it was the poppet that wasn't sealing on the seat.... I pulled the gun all apart, and on inspecting the seat with a loupe I saw a very small nick on the sealing surface, and a corresponding mark on the soft seal insert on the poppet.... The brass seat was easy to polish, but rather than take a chance on the seal in the poppet I decided to replace the poppet head with a Delrin one which is more suitable for HPA use.... I drove the stem out of the old poppet and reused the stem, and made a new poppet head from a piece of 3/8" Delrin.... Since I didn't have to worry about the OD of the brass poppet, I was able to turn the entire head down to 0.32", which is the diameter of the valve spring, and 0.050" smaller than the original.... I lapped the new poppet against the repaired seat, and reassembled the gun....

The first test shots, with the RVA set at 4 turns out as before, were a bit of a shocker.... Instead of getting 847 fps (22.8 FPE), the velocity was in the high 880s with the occasional 890+ (25.0 FPE).... Obviously the new Delrin poppet was a big improvement.... Since the throat and seat is still the same size, I figure the increase must be from the decrease in head diameter and the corresponding easing of the flow around it.... There was another big change, and this one I expected.... I was able to reduce the hammer spring preload another 2 turns to get back to the "knee" of the velocity curve.... This is because the harder seal material compresses less, and that reduces the amount of hammer energy required to "unstick" the valve from the seat.... It sure is nice to see theory backed up by results....

Bob

rsterne · Moderator

Here is the results of preliminary pellet testing indoors at 20 feet....

I shot 5 shot groups from the bench, and then sorted the results by using two washers of 3/8" and 1/2" diameter.... If the 3/8" washer completely covered the group, it is coded green above.... If the 1/2" washer would NOT cover the group, it is coded red.... All others (3/8" to 1/2") are left unmarked.... The best group was the 15.9 gr. JSB Exact Jumbo, which was just a ragged hole, closely followed by the other JSB Exact Series pellets....

The Ballistics Coefficients listed are taken from testing over 25 yards with another gun that shot about 1 FPE more muzzle energy, so they will be very close in this gun.... Pellets shown in blue will be tested at longer range....

Bob

rsterne · Moderator

Today I added a Hatsan Air Stripper to the barrel and did a few tests with the JSB Exact 15.9 gr. pellets which produced the best groups yesterday.... I had never tested a stripper before, so I set the gap between the stripper cone and the barrel at maximum (about 2 calibers gap from the muzzle) and then moved it in about 1/2 cal. for each group.... I shot 5 shot groups through the Chrony with the target still at 20 feet, not knowing what to expect....

Without the stripper, the velocity was 849 fps, and with the gap at maximum it seemed to be a couple of fps less.... Each time I moved the stripper closer to the muzzle it picked up a couple of fps, and by the time the gap was down to just over 1/16", the velocity was 855 fps.... I figure that when the gap is small, the stripper is focusing some of the muzzle blast behind the pellet and effectively lengthening the barrel a fraction, although not very efficiently.... The group size did, in fact, seem to vary as I adjusted the gap, although at this short range realistically the changes were very small.... I expect it would be easier to see results at longer ranges.... Since the smallest group seemed to be with a gap of just under 1/4", I decided to adjust the gap to exactly the same size as the head of the pellet (just because) so I clamped a pellet in some forceps and them used the head like a feeler gauge and adjusted the stripper cone so that the pellet just touched it and the muzzle.... Here is the 5-shot group I shot at that setting....

That is a .22 cal JSB Exact sitting in the hole.... The target won't meet the Feinwerkbau test where a pellet will hang in the hole with the target vertical, but it won't fall through either.... I figure the group is 0.03 - 0.04" C-T-C, or about 0.25"-0.26" OD.... In any case, I'm pretty pleased with it, and indeed the stripper seems to help....

Bob