Air Gun Home :: View topic - Non-PAL QB 79 on HPA

rsterne · Moderator

There have been lots of tuning threads on the QB 78/79.... but I actually haven't seen anything recently.... I have NEVER seen one aimed at producing a non-PAL PCP based on either, and after several questions on the CAF over the past few months about the availability of a relatively inexpensive non-PAL PCP, I decided to have a go.... I selected the QB79 as it is already set up to use an HPA tank/regulator setup with the typical paintball CO2 output.... The tank I picked is the 3000 psi 13 CI Ninja paintball tank which comes stock regulated at about 850 psi.... I chose this tank for several reasons.... It is relatively small and lightweight and will clear the barrel.... The regulator uses a coil spring and has shims that can be removed to reduce the pressure to keep the velocity under 500 fps... It has a 1.8K burst disc fitted to the downstream side of the regulator so that if it fails, the disc will prevent the gun from seeing over 1800 psi.... Since CO2 can reach 1900 psi at 120*F that means that the stock mounting system for the tank block should be safe, especially since there are two additional screws through the stock into the tank block as well as the two in the tube.... I feel confident that if you only fill the tank when the action is bolted into the stock there is no more chance of failure than on CO2....

The first part of this thread will deal with the disassembly, deburring, trigger tuning and lubing and can apply to any QB 78/79 series.... I will NOT be doing any performance mods to the gun, as the whole point is to build a non-PAL PCP.... In fact, I will not even be removing the valve or tank block, so that the seals are not disturbed.... I will be testing the gun in both .177 and .22 cal.... with the hope of giving a recommended regulator pressure / shim setup for each caliber that will keep the gun under 500 fps with "normal" weight pellets.... There are two ways to install the tank on the QB 79.... You can either remove the barrel and rotate the tank.... or you can remove the gauge and fill fitting from the tank in which case you don't have to remove the barrel.... Since if you disturb the gauge and fill fitting you chance a leak.... and removing the barrel is necessary for deburring.... I chose to leave the regulator fittings undisturbed.... My starting point was a brand new QB 79 from Eric at http://www.scopesandammo.com and a Ninja tank purchased on eBay.... Other than my time, I have under $300 invested in the gun, including shipping costs and the second barrel and bolt.... Let the project begin....

rsterne · Moderator

The first step was to disassemble and deburr the breech and the back half of the main tube.... I did it this way....

1. Turn the safety button to the ON position and carefully drive it out towards the lever side
2. Remove the two screws at the front of the stock and the main stock retaining nut
3. Remove the action from the stock
4. Remove the pinch screw and nut from the barrel band (don't lose the nut)
5. Remove the barrel set screw and slide out the barrel (the O-rings will likely be shredded)
6. Remove the rear trigger mounting screw, and both rear plugs
7. Remove the front breech screw located under the bolt
8. Lift off the breech and remove and set aside the transfer port seal
9. Remove the two pins from the hammer and the cocking sleeve
10. Slide the cocking sleeve, hammer spring, and hammer out the back of the main tube
11. Loosen the set screw in the back of the bolt that retains the bolt handle and remove the handle
12. Slide the bolt out the back of the breech

At this point, your gun should look like the photo below except the trigger group is still assembled.... I forgot the transfer port seal in the photo.... Both the .177 and .22 bolts are shown....

At this point I deburred all the holes and slots in the breech and the rear portion of the main tube.... Pay particular attention to the inside of the transfer port and setscrew holes in the barrel portion of the breech.... If you don't do a good job, you will destroy the O-rings on the barrel (circled in yellow, above) every time you slide it in.... The two closest to the back of the barrel likely got chunks torn out of them when you pulled the barrel out, if so, toss them.... They are a standard #012 O-ring (1/2"OD x 3/8"ID x 1/16"T) and I would recommend you purchase a few.... Make sure there are no sharp edges on any of the slots, and also on the milled flats on the bolt.... Eliminate anything that can cause binding or wear.... Carefully clean out the inside of the main tube and breech, and the bolt.... Make SURE there is no grit or dirt remaining.... Lightly oil the inside of both, a thin even film all over to prevent corrosion and provide lubrication.... 30W non-detergent motor oil or Pellgun oil is OK, although I use "Moly E" straight from the can, which is a suspension of Moly in oil.... I use that for nearly all moving parts on my airguns.... Check that the main mounting pin and the valve securing screw in front of it are tight.... mine were both loose.... Also check the two screws retaining the tank mounting block....

Now turn your atttention to the hammer.... Polish the large diameter sections starting with 220 grit and ending with 600 grit.... Small holes or pits aren't an issue, but the part that rubs on the inside of the tube you want smooth... Make sure that the pins fit smoothly into the holes (the one with the square top goes into the cocking sleeve).... When the trigger is pulled, the hammer MUST fall freely forward under it's own weight without the spring pushing it.... You will notice that when you pull back the hammer, it cocks the sear.... When the sear is cocked it is in the position shown.... You can observe the interaction between the trigger and the sear through the large hole in the side plate.... Now that we've gone this far, however, let's improve the trigger....

Remove the two screws holding on the side plate, and carefully pry the plate off.... You will see how the trigger works, allowing the sear to rotate (clockwise from the position above) to fire the gun.... There are three adjustment screws in the back of the trigger housing.... The upper one adjusts the amount of overlap between the trigger and the sear, and governs how far the trigger moves before firing.... The middle one acts as a stop for how far the trigger moves after firing.... The larger, bottom one adjusts the tension on the trigger spring.... I feel that the stock trigger spring (circled) is too strong, and the replacement (also circled, not yet installed) is shown.... The replacement is 0.020" wire instead of 0.027", and about the same length.... The other item circled is a small spring loaded plunger that can be used to provide a 2nd stage.... It replaces the middle stop screw, and I'll give the details in the next post....

Bob

rsterne · Moderator

Here are the modifications I did to the trigger.... The instructions below detail what I did.... but you can do any or all (or none) of these....

1. Polished the sear/trigger contact surfaces
2. Installed spring loaded plunger to provide a second stage
3. Replaced trigger spring with lighter one
4. Adjusted the trigger to my preferences

Note how the parts are installed and make a diagram, photo, or refer to the photo in the post above for orientation.... Start by backing off the spring adjustment screw and then pop out the trigger and spring.... Carefully lift the top half of the sear spring out of the way and remove the sear.... Polish the surfaces where the trigger and sear meet, starting with 220 grit and finishing with 600 grit.... Small pits are not important but you want the surfaces to be smooth so that they will slide over each other easily and not drag.... All edges should have any burrs removed, but do NOT change the shape or angle of anything or you may make the trigger dangerous.... The areas to be polished are circled in yellow in the photo below....

I replaced the adjustment screw which was in the middle (location 2) with a small spring-loaded plunger (circled in red).... I purchased it from KBC Tools, and the part number is 1-903-52001 on page 793 in their current online catalogue.... http://www.kbctools.com/can/Navigation/NavPDF.cfm?PDFPage=793

This was not my idea, I borrowed it from a person from a couple of years ago (sorry I don't remember who, but thanks).... The plunger is 6-32 thread, so the hole in the trigger frame will need to be drilled 7/64" (or #36) and tapped to 6-32.... Make sure you remove all shavings from the drilling and tapping and insure there is no dirt or grit inside the trigger frame or on any moving parts.... The travel on the plunger is small, and the force to move the pin is relatively large, but because of the leverage, it provides an nice second stage providing the spring tension on the lower adjustment is not set too high.... That is why I recommend using a lighter trigger spring.... The plunger is difficult to install for two reasons.... It has a hardened drop of loctite on it to provide drag.... and there is no slot in the outside end for a screwdriver.... I scraped the loctite off with an X-acto blade and I ground a slot in the end with a Dremel cut-off wheel.... This destroys the tiny adjustment screw, but I never used it anyway.... Once you get your final adjustment, you may need a tiny drop of blue loctite to hold that adjustment (I didn't).... The numbers in the above photo indicate the stage adjustment screws and the spring tension adjusment.... The lighter spring is shown installed above.... It is 3/16" OD x 5/8" long and made from 0.020" wire, Trakar part # C0457-051-0158, but any similar spring will do as there is an adjustment screw to set the tension....

When reassembling the trigger, I used Honda Moly 60 for the pivot holes and pivot pins, the ends of the springs, and the sear/trigger contact points.... You don't need much, just a thin film will do.... Here is a photo of the reassembled trigger showing it adjusted the way I liked it....

Note that the sear overlaps the trigger by about 2mm.... You adjust the upper (first stage) screw to achieve this.... Backing out the screw will provide a longer first stage travel and vice-versa.... Note that there is a small gap between the back of the trigger and the tip of the spring plunger.... As you pull the trigger, the sear overlap decreases, and the object is to have the trigger contact the second stage adjuster (the plunger) just before the sear releases.... Don't go crazy trying for a hair trigger, I set mine up for about 2/3 of the sear travel on the first stage and the last 1/3 on the second stage.... If you don't install the plunger (and therefore only have a single stage) I would reduce the sear overlap to about 1.5mm (1/16") or so.... For safety, you should not reduce the trigger spring tension too much.... Ideally, if you pull through the first stage and release the trigger with the hammer cocked, the trigger should return to the original location.... If you reduce the spring tension too far, that won't happen and the gun could fire when bumped.... not a good idea.... Replace the side plate, and you're done with the trigger mods....

More to come....

Bob

rsterne · Moderator

Now we're getting ready to reassemble the gun.... This is basically the reverse of disassembly.... using the proper lubes, of course.... I applied a small amount of Honda Moly 60 to the bolt and the hammer and burnished (rubbed) it in until there was just a thin, even film on all the contact surfaces.... I applied some to the spring, inside the hammer, and on the spring guide, and twisted and turned the parts until there was again a thin even film on the guide, the spring, and the hole in the hammer, wiping off the excess.... Likewise, I coated the cocking pins and their holes as the Moly makes a great anti-sieze agent so the pins don't get stuck.... I then slid the hammer, spring, and guide/cocking slide into the back of the main tube, and added a couple of drops of Moly Slip E (or oil) through the slots to lubricate the hammer and cocking slide.... Then I installed the rear cap, slid in the rear trigger/breech screw, and installed the cocking pins (square one in the rear cocking slide).... I slid the bolt into the breech, added a couple of drops of Moly E (or oil) through the slots, and installed the bolt handle and tightened the setscrew, and slid in the rear cap, threads to the bottom.... Here are the two subassemblies....

Carefully install the transfer port seal with the small side up towards the breech (dry, no lube).... The hole in the valve is quite often off center in the hole in the tube.... If you inspect the seal you will be able to determine which way it was in originally, and refit it in the same orientation.... That willl prevent damage on reassembly.... Put a dab of Moly 60 on the heads of the cocking pins and carefully position the breech on the main tube.... Make certain that the transfer port is in place, and that both cocking pins engage the slots in the bolts as circled above.... Then screw in the rear screw loosely, install the front screw, double check the alignment of the transfer port, and evenly tighten both screws.... The gun will now look like this....

We're ready to install the tank.... You don't want to use a flammable lubricant where there is high pressure air present, and there are better lubes for O-rings in any case.... I use Dow Corning "Molycote 55" which is a silicone O-ring lube, but you can also use "Diver's Grease" or other silicone lubricant.... Put a generous film on the O-ring (circled in red) and the threads but not globs.... Thread the empty tank into the tank block and see where it stops.... Ideally, either the gauge or the fill fitting should point straight down, but as long as they don't interfere with the barrel it's not critical (other than for appearance).... Mine stopped with the fill fitting nearly straight down, which was good enough for me.... If the tank tightens up in the wrong place you can leave it and when pressurized it won't tend to wiggle much.... A better solution is to purchase a "shim kit" from Ninja which fits between the end of the tank block and the shoulder on the regulator.... They clip in from the side and you select the proper thickness to get the tank to tighten up where you want it.... Alternately, you can use shims inside the tank block so that the end of the regulator stops against it.... but make sure they don't block the air passage.... Once you have installed the tank, your gun should look like this....

Put about 1000 psi of air in the tank and set it aside (overnight is best) to check for leaks.... If the pressure drops, test for leaks using soapy water and a small brush.... and repair them as necessary....

Nearly there....

Bob

rsterne · Moderator

All that remains (assuming there were no leaks) is to reinstall the barrel and put the stock back on.... It is probably a good idea to clean the barrel first, especially if it's new.... The weed-eater line technique and a few patches with Goo-Gone and then dry patches until they come out clean are a good method.... Use the Dow 55 (or other silicone O-ring lube) on the barrel O-rings, and a thin film in the front of the breech as well.... Slide the barrel through the barrel band, and position the barrel with the set-screw recess at the top and PUSH IT STRAIGHT IN - DON'T TWIST IT into the breech.... If you twist it, you are much more likely to damage the O-rings.... Once the barrel is seated to the shoulder, install the setscrew and snug it into the recess.... Twist the barrel a tiny amount back and forth as you tighten the setscrew into the recess to insure it's properly oriented....

Slide the action back into the stock, insuring that the barrel band slides into the notch in the stock.... Reinstall the two front stock screws and the main retaining nut, and reinstall the safety.... You slide it in from the right side, with the arrow pointing to the "S" (ie safety ON).... Lift the plunger to get it started and then push it in while wiggling it.... You should not have to drive it into place.... Put the nut and bolt back into the barrel band and snug them up.... Your rifle is now ready for initial testing.... (Yeah, I didn't install the safety)

I assembled the .177 version first, with a stock regulator.... and actually got a bit of a shock.... The 7.8 gr. JSB Express pellets screamed across the Chrony at just over 650 fps.... I continued shooting until the velocity dropped to just under 500 fps and then carefully refilled the tank until the check valve clicked, indicating that the tank was at that pressure.... The pressure was (are you ready for this?).... about 350 psi.... I don't even know if I can adjust the regulator that low.... but before I try, I'm going to test the .22 barrel and see what happens....

Stay tuned....

Bob

rsterne · Moderator

I installed the .22 cal barrel and bolt, and the initial testing with stock regulator pressure (~850 psi) was just under 550 fps with 14.3 gr. JSB Express pellets.... I shot it down until the velocity dropped below 500 fps, and the pressure was about 650 psi, so that was my goal.... OK, so now it's time to reduce the regulator pressure to get the velocity under 500 fps.... I removed the barrel and unscrewed the tank.... You can do that with the tank under pressure provided the regulator output is under about 1000 psi, over that I wouldn't recommend it.... There is a vent hole in the tank block and just after it is unscrewed far enough to seat the check valve, the O-ring uncovers the hole and the air in the gun (but not the air in the tank) vents.... If you're not expecting it, it will scare the heck out of you.... Just go slow, and when it starts to vent wait until it quits then unscrew it another 1/4 turn and make sure it's completely vented before you take the tank off completely.... Alternately, you can shoot the gun down to zero, and if you don't have a way to dump the remaining air in the tank you will have to do that, as the tank must be completely empty before you can take apart the regulator....

The Ninja tank comes with an allen wrench to undo the setscrews in the "bonnet" and a long screw of the same thread to use as a handle to loosen the bonnet.... Remove the setscrews completely, screw in the long screw until it stops, back it out 1/4 turn so the tip doesn't drag on the regulator body and then unscrew the bonnet 1/4 turn ONLY.... Remove the long screw and continue to unscrew the bonnet by hand.... If it won't turn easily, there is probably still some air in the tank, and you MUST dump that air before proceeding.... The reason for removing the long screw is that if it is sticking through the bonnet it will hit the threads on the aluminum regulator body and ruin them, making removing the bonnet difficult if not impossible.... I speak from experience....

Once the bonnet is removed, you will find the following parts inside....

Left to right: the tank and regulator body.... two silver shims (0.005").... two red shims (0.021").... piston and spring.... pin valve.... bonnet.... Also shown are the allen key, long screw for leverage, and the two setscrews that hold the bonnet in place.... Inside the piston there is also a small spring for the pin valve.... It’s not in the photo, but be careful not to lose it....

There will be two red shims, but the number of the thinner silver shims may vary as they are used to "fine tune" the regulator to ~850 psi.... I have done a bit of playing with these regulators in the past, so I guessed that removing one of each thickness shim would be close to what I wanted.... I reassembled the regulator with only one silver and one red shim, filled the tank to 1000 psi and checked the output.... It was ~670 psi, pretty close to what I wanted, so I reassembled the gun and checked the velocity.... The first shots over the Chrony were 484-494 fps, averaging 490, but that was only with a 1000 psi fill.... It is common for the regulator output pressure to creep upwards very slightly as the tank is filled more, so I filled the tank in 500 psi increments with the following results:

1000 psi down to 500: 32 shots, averaging 490 fps, 484-494
1500 psi fill, 10 shots, 497 fps avg., 489-500
2000 psi fill, 10 shots, 500 fps avg., 496-504
2500 psi fill, 10 shots, 501 fps avg., 496-504
3000 psi fill, 10 shots, 504 fps avg., 497-508

The velocity was just a little too high, so I drained the tank, took the regulator apart again and removed the second silver (0.005") shim.... That dropped the pressure to ~635 psi.... Of course that was too much, and with a 3000 psi fill, 10 shots averaged 487 fps, ranging from 480-492.... I took the regulator apart yet again, and added a 0.002" shim and ended up with 650 psi.... About now I was wishing I had trusted my original plan.... I refilled the tank to 2950 psi (all the air I had) and shot two 32 shot strings.... The first averaged 491 fps (487-495) and the second averaged 489 fps (485-492).... The pressure at the end was 2000 psi, so I used 950 psi for 64 shots averaging 490 fps which is 7.63 FPE for a total of 488 FPE.... The air used was 950 / 14.5 = 65.5 bar x 13 CI = 852 CI.... The efficiency worked out to 488 / 852 = 0.57 FPE/CI which is pretty low.... However, even with that, I should be able to get about 160 shots per fill.... I didn't check the total number of shots, or the velocity at the 650 psi regulator setpoint.... but I would expect the velocity to be about 480 fps at the end of the string....

This particular regulator takes about a 0.001” shim change to change the output pressure by roughly 7 psi.... That in turn, changes the velocity by about 3 fps.... Another regulator and a different gun might not react in quite the same way.... so if you decide to follow my lead and build a non-PAL QB 79 on HPA be prepared to play with the output pressure to keep the gun under 500 fps.... Having said that, in .22 cal, you could probably remove one red shim from the regulator before initial assembly.... and then fine tune after that if necessary....

I looked at the pressure curve for this regulator, and it looks like the minimum pressure I can achieve is about 500 psi or a bit less.... I can't get to the roughly 350 psi needed to keep the .177 version under 500 fps without making other changes, so at the moment I don't plan to pursue a non-PAL in .177.... I have been thinking about the rather low efficiency, and it kind of fits with the fact that higher pressures tend to lead to higher efficiencies.... The QBs have a very strong hammer spring, and I'm wondering if that, in combination with the low pressure, is lowering the efficiency.... It is possible that running a higher regulator pressure and a lighter hammer spring might result in an increased shot count.... On the other hand, it might just mean the same shot count but less air used.... or it might not be any better.... In any case, I think this project has come to a successful conclusion.... I have made possible a non-PAL QB 79 in .22 cal, running on HPA, that will deliver about 160 shots per fill.... All in all, I think that's a success....

Bob

AirGunEric · Site Admin

rsterne · Moderator

Yes by "stock" I meant paintball stock.... 850ish psi....

I would have no problem running a QB up to 1500-1600 psi PROVIDING you have a 1.8K burst disc in the system.... I've gone a bit more, but you need a heavier hammer spring to do that.... One of the issues is whether or not the steel sleeve behind the valve is right up against the valve and sharing the load or not....

I haven't tried more pressure in this gun.... yet.... I have a QB 78 that is currently getting 32 FPE (18 gr. @ 900) at about 1600 psi but the mods are extensive and I haven't put a tank on it yet.... I may swap the parts between the two guns just to see what I lose with the small volume between the tank and the valve on the QB 79....

Bob

SMP · Silver Status Member

Absolutely brilliant write up Bob.

Very good info for anyone wanting to run a pcp setup without jumping through the hoops to get a PAL. It should be achievable by anyone.

As you know I went the other route with extensive machining and ended up at the same result. Albeit for different reasons.

I think it's important to mention that a person with a QB78 with a bulk fill cap could achieve this result by ponying to a tank with a paintball hose and following all your steps for regulator adjustment. I use this setup when I want to have a long practice session.

rsterne · Moderator

I was not very happy with the dismal efficiency of this gun, so I decided to try a trick that works in non-regulated PCPs and I'm finding out is very important in regulated ones as well.... I lightened up the hammer hit....

I played with preload by deactivating the "cock on open" function and shimming the sliding block.... Once I determined that, yes, in fact there was a large gain in shot count to be achieved with less hammer strike I tried a couple of lighter springs, but didn't get the desired results.... What I found that was key in this particular gun was to have no, or virtually no, preload on the end of the valve stem after the gun fired.... and yet a stiff enough spring to achieve the necessary hammer velocity.... It turned out that all I had to do was clip 1.5 coils off the stock spring.... and return the gun back to "cock on close"....

The velocity throughout the string is virtually unchanged.... The gun still shoots about 496 fps at 3000 psi.... gradually decreasing to about 484 fps at 1000 psi.... However, instead of only getting about 33 shots per 500 psi drop in pressure, it now gets 50.... YUP, 50% more shots of the same power.... Mr. Green

There is one other small benefit to the lighter spring.... You can now shoot the gun down to about 500 psi (150 psi below the regulator setpoint) before the velocity drops consistently below 480 fps.... That means that the total shot count is now about 250 shots at 7.6 FPE from a 2500 psi (172 bar) pressure drop on the 13 CI tank.... That means I'm getting 1900 FPE from 2240 CI.... and the efficiency is increased to 0.85 FPE/CI.... from a previously dismal 0.57 FPE/CI.... I'm now VERY happy with this HPA conversion....

Bob