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All Springer/NP/PCP Air Gun Discussion General > Show Off Your Air Guns With Mods (SHOW and TELL)

Non-PAL QB 79 on HPA

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rsterne:
The QB 79 adapter block works fine in a 22XX tube.... but there is a potential safety concern if the regulator isn't fitted with a 1.8K burst disc.... The screws that attach the block to the tube are VERY close to the end of the tube, and the stock QB screws are quite a soft material.... I know of one case where a regulator that didn't have any downstream burst disc failed, pressurizing the QB 79 main tube to 2600 psi.... The action was not installed in the stock at the time.... The screws sheared off, and the tank block/regulator/tank assembly blew off the end of the gun and went spinning across the floor of the guys shop.... Fortunately, nobody was hurt, although I think some underwear suffered an indignity or two.... On inspection, the holes in the end of the main tube were nearly torn through as well.... When you consider that CO2 can reach 1900 psi at 120*F, IMO there is insufficient safety margin in the design when use on a 22XX.... IF the QB stock is installed, there is an additional safety aspect as there are two more screws going through the stock and into the tank block.... Also, there is a barrel band clamping the barrel to the outside of the block.... These additional features may (or may not) add enough of a safety margin, I simply don't know....

When I use the QB 79 tank block in a 22XX tube, I add a 3/16" shear pin and an 8-32 SHCS with the head in shear IN ADDITION to upgrading the original 4mm screws to high tensile ones.... Here are photos of one I have modified....









JDS Airman, who sells tanks and regulators, is now marketing what to me appears to be an upgraded QB 79 tank block....

http://www.jdsairman.com/newproducts.html

With either my version, or his, you can only use the inner O-ring, as the new screws / shear pin are inboard of the original outer O-ring.... This is, fortunately, not a problem, because the outer O-ring only works if the inner one leaks....  ;D

In order to convert a 22XX to run HPA, you must use a Disco style valve, sealed with O-rings, and properly secured in the main tube, of course.... Some of the 22XX tubes have a safety roll-stamp in front of where the Disco valve O-rings would be, and that area MAY be questionable from a safety point of view.... One chap I know hydro tested a 2240 tube (including the roll-stamp area) to 10,000 psi, and all that deformed was a slight set in the tube around the head of the valve securing screws (three 8-32 low profile SHCSs with the head set down 0.030" into the valve).... What we DON'T know is how deep the roll-stamp was.... I've personally seen them so shallow you could hardly read them.... or so deep they distorted the INSIDE of the tube.... To me, the latter is like the pineappling on a grenade.... I designed an extended valve front end that moved the O-rings out past the roll-stamp on a 2260 (which is the longest).... It can also be used in a Disco to cover the gauge port hole and eliminate the gauge.... Muzzle Mack built them for a while, but now he is out of business.... Here is a photo of one I made with a gauge port....



That is the 2260 tube from my Grouse Gun, and you can see that the O-rings are ahead of the roll-stamp.... I use a 2000 psi fill in that gun, just like a Disco.... I have used my modified QB 79 tank block on a couple of guns similarly equipped, running a 1600 psi regulator and a 1.8K burst disc.... and I can get 32 FPE and about 45 shots per fill on a 13 CI Ninja tank.... Here is an example....



That particular version uses a home made tank block with a short weaver rail built in for a Bipod.... not a QB 79 Tank block, however....



The base gun is a 2260 with the stock and main tube shortened.... I made a simple, low bipod by bolting a bar to the bottom half of a Weaver scope ring and fitting a couple of bolts and rubber caps.... Pretty crude, but it works great.... To get 32 FPE requires some port work, of course.... Without it, you can duplicate Disco performance (23 FPE) but at about twice the shot count.... and a VERY flat shot string.... Take your pick, more power = fewer shots, as usual....

Bob

rsterne:
I was not very happy with the dismal efficiency of the QB 79 non-PAL PCP conversion, 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....  ;D

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

lloyd-ss:
Bob,
Good discussion about efficiency.  I imagine when you are getting different efficiencies you are actually altering the shape of the air pulse.  Maybe the valve just takes longer to close on some of those wasted-air shots.  I have heard about hammer bounce, but have only really encountered it once in a very severe condition.  It was almost as if I had tuned the setup to operate at one of the harmonic frequencies.  I could almost adjust it to drain the tank in 3 shots.

Bob, have you ever played with the valve closing spring much? That seems ripe for experimentation and seems like it ought to be almost as important as the hammer spring.  What do you think?
Lloyd

rsterne:
I've never played much with the valve spring other than I did put in a very light one once in an HPA and ended up with a machinegun that drained the tank in one shot.... On a low pressure (eg CO2) system it is quite important, I think less so as the pressure increases.... Of course a heavy enough spring could alter the shape of the bell curve, or even eliminate it if the spring was heavy enough I guess.... If the spring force was, say, 10 times the force from the air pressure, and then you had a mondo hammer to open it I would think the first shot would be the fastest (in a non-regulated gun) and then just decreasing from there as the valve duration would stay relatively constant (because the spring is most of the load) and the amount of air released would decrease in  proportion to the pressure....

I figure that is the spring if only heavy enough to close the valve for filling, then getting the valve to be self-regulating is everything.... At the other end (really heavy spring as above) there is basically no self-regulation.... I guess that somewhere in between there might be an optimum.... but is it worth trying to find it?.... I dunno....

I've only experienced hammer bounce in guns like a 2240, which is the classic "fa-r-a-r-a-r-t" sound they make.... Some people use an HDD, I just decrease the hammer spring hit to save gas and the bounce goes away.... I guess if you take that, plus my experience with the "machinegun", in both cases the hammer spring was strong and the valve spring weak.... Maybe a pattern there, but I don't have enough data.... The other hammer I had run wild was the Hayabusa when there was no air vent in the tube between the hammer and the back of the valve.... drilled a hole to vent that area and it cured it....

Bob

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