Canadian Airgun Forum

Saw the QB78 synthetic and thought it would make a great project gun for a bottle-fed HPA conversion.

So far not much done... I didn't bother trying it with Co2, I just tapped the front end cap and attached it via remote fill adapter to the bottle, which is presently zip-tied to the underside of the stock until I decide exactly how I'll mount it.

I haven't measured the regulator output, but I'm guessing around 1400 psi? I've got a 1.8k burst disk on the output side, so it can't be much more than that.

So without any valve, port, or hammer mods (I haven't even opened it up to remove the felt filter from the valve), it shoots JSB 18.1's at 545 fps. Not bad! So now what?

I'm hoping to tune for about 800fps with 18.1g's.

I'll open the sides of the valve to take advantage of the 50cc tube for plenum. Drill out the exhaust port and probe.

Thoughts on transfer port? I've got 1/4" poly tube that I've been meaning to try. Do I really need to bore out 1/4" shoulders for it to sit on, or can I get lucky and just squish it in there? I've got a Crosman transfer port that's been opened up, but it's looking a little rough.

I didn't realize an RVA would be such a headache for this build, I may just get a couple springs and clip coils until I get it right.

Anyone have experience using the bstaley o'ring mod behind the valve? I know this is mostly for unregulated PCP's, but I've read about some people improving efficiency with this...

Anything you wish you did with your QB78? Looking forward to this build.

There are a lot of un sure here. And un sure isn't a great idea when dealing with HPA.

If you picked up the HPA tank from a paintball place, it would be only 800 PSI. If you have no idea what the output pressure is, Maybe you should find out.

Also there was a guy that posted on GTA blowing up his HPA QB 78 trying to push it too much.

The 1400 psi isn’t a total guess, it’s based on observing where the regulator falls off on another build, as well as the FPE increase after shimming it from 850 psi. I would like to build a gauge/slide check line to measure the output exactly.

The qb that blew up was up around 2200 psi (at the least), and failed at the drop block retaining screws. I’m keeping the original qb78 endcap, which shouldn’t have a problem at these pressures. I’d consider pinning the endcap since I will only have to remove it to get at the valve, but I should be alright.

The valve and end cap are only safe to around 1800psi and I think the tube isn't rated much higher. You could pin the end cap but you'd probably have to make another oring groove further up in order to get the screws into the thick part of the tube.

Anyway, you can easily get 800fps without going past 1800psi. I'd stick with your 1.8k burst disk and keep it at that 1400-1500psi range. This is plenty to get 900fps in 22 with 15.89 gr, if you open up the valve on the front, and put a new transfer port seal in. I just used the black rubber seal from the crosman 1322, it's just a drop in change, no cutting flats in the valve port needed. I did change out the valve stem and poppet, for a delrin one from Gipetto. He may still have some in stock you could buy. It makes it a lot easier cocking, because you need less hammer spring power to crack open the valve, and gives more power for less spring. The Bstaley mod worked great for me when I had the unregulated setup. I haven't compared efficiency now that I've got a regulated bottle, but I still have the oring in, and at 1300psi I'm getting 865fps average or so on my rifle. I thnk the only other things I really did to change velocity, was to open up both holes on the bolt probe as big as I could, and that's about it. I did change the hammer and change it to cock on open, but it was able to shoot as fast before switching those, only my efficiency improved with a lighter hammer.

Best of luck, and don't push the pressure! Keep that 1.8 burst disk and we'll all breathe easier for you!

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Thanks guys. There’s no plans to ditch the 1.8k disc or up the pressure.

Looking forward to how it turns out for you!

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Update:

I opened up the sides of the valve, removed the peircer, etc... the valve spring is still resting on the black washer, so still pretty restricted. I also put in a .25” poly transfer port, and drilled out the bolt probe.

Went from 545fps to 730fps with 18’s. I don’t think i’ll open it up much more, everything sealed first try, so that was a nice surprise.

Efficiency’s still got to be worked on, I’m not sure if I should start clipping coils off the hammer spring, or start experimenting with o’rings behind the valve.

Accuracy is so so... I’ve only put 70 or so pellets through so far, but it’s still opening up about an inch at 27 yards.

A really rough estimate of my efficiency puts it around .48fpe/ci

So I think I'm safe clipping a coil or two...

Should have been patient before complaining about accuracy, it's now clover-leafs at 27 yards. Getting exciting!

My regulator puts out a higher pressure when the bottle's full compared to say 1400psi, but if I shoot from 3000 down to 2000, I keep all my shots between 725-738fps. Right now that's only about 28 shots, I'm hoping to stretch that out to 40 or 50 without losing much fpe.

I'd definitely cut a few coils. And try the bstaley orings =)
I was getting 1.5fpe/ci or over with mine so you definitely should be able to get a lot more shots and efficiency than that.

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That's great it's shooting accurate anyway. [emoji846]

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I would say you definitely have too much hammer strike if you are wasting that much air.... try cutting the spring.... The bstaley O-ring mod is intended for unregulated PCPs to flatten out the shot string.... Reducing the hammer strike, particularly in conjunction with using a gap between hammer and spring (either an SSG or SSS) is much better at improving the efficiency on a regulated PCP....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

Seems a little strange that it's wasting that much air at 1400psi. I'd be interested to see it checked by a proper pressure tester, since it should be getting better efficiency than that you'd think. Unless maybe you have a major leak at the transfer port? Have you tried putting a kleenex over the breech area when you shoot to see if any air is blasting out somewhere?
What do you think Bob, would the stock set up have that much extra hammer power at 1400psi?

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The only way to tell is to try different hammer spring preloads via shimming, cutting coils, or an RVA.... Only doing that will tell you if you are up on the plateau (where more hammer strike only wastes air without increasing velocity).... I do agree that a stock hammer spring setup would be unlikely to have enough dwell to cause a dump shot (valve open after the bullet exits) however.... which is the usual cause of such low efficiency numbers.... A leak may be the culprit....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

Clipped a coil off the hammer spring, no change at all in velocity. What a pain to get at the spring, might take 2 coils off next.