Author Topic: Two 88 gr. CO2 Cylinders for QB78 HPA Conversion  (Read 1364 times)

rsterne

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Re: Two 88 gr. CO2 Cylinders for QB78 HPA Conversion
« Reply #15 on: January 15, 2018, 09:50:46 AM »
As promised, here are the results.... First, the data showing velocity vs preload (or gap) on the hammer spring, at 1800 psi, with the two pellet tested, the 18.1 gr. and 25.4 gr. JSB Exacts....



I was delighted that the new PEEK poppet with the small head and small stem was so easy to open.... Of course the 50% increase in hammer stroke would have helped a lot to make up the difference between CO2 pressures and the 1800 psi I am running on this HPA conversion.... Still, I am only using a stock QB hammer spring, and even with 1/16" of gap (2 turns) between the spring and hammer when uncocked the gun is able to reach maximum velocity with this combination of porting and pressure.... The downslope of the velocity curves are very steep (it doesn't take much of an increase in gap to drop when below 800 fps).... and the knee of the tuning curve is very pronounced.... making adjustment of the gap quite sensitive.... Just 1/2 turn can move you from a perfect tune to starting at the peak, or starting to get valve lock at the beginning of the string, with a big velocity drop.... I played a bit with the adjustments and came up with the following strings, starting from an 1800 psi fill....



The performance is very similar to the Beeman Chief I just built, except for a lot more shots from the larger reservoir (290 cc instead of 135 cc).... The velocities are just slightly lower than the Chief, a result of the slightly lower fill pressure (1800 vs. 2000).... I don't think the larger porting on the Chief actually made any significant difference, and I am wondering if the 3/32" bolt probe, which was shared by both guns (which are both .22 cal), is the limiting factor to the flow.... In theory the area around the probe in the barrel (chamber) is the equivalent of a hole 0.198" in diameter, and on the Chief all the ports are larger than that.... while in this QB conversion the transfer and barrel ports are only 0.172".... It may be that just as we like to see the throat area (after the valve stem is deducted) a bit larger than the porting.... the same thing applies to the chamber and the bolt probe.... I know that using a retracting bolt to completely clear any obstruction from the air path in the barrel helps performance in bullet shooters, where we are after full bore area porting.... It may well be that once you go larger than 80% of bore for the transfer port (which is as large as you can go with a drilled barrel port without possible loading problems).... that to see full benefit you have to retract the bolt out of the airflow.... With a strictly pellet shooter, I could probably get away with a smaller probe (1/16") and still seat the pellet fine.... but with a bullet shooter (which I may do with the Chief) such a small probe would sink into the back of the bullet, because you have to push harder to chamber them.... In any case, a smaller probe would not likely change anything on this build, with the porting less than 80% of the cailber....

The astonishing thing, for me, is the efficiency of this gun.... The total reservoir volume (88 gr. cylinders at 121 cc each plus the QB78 air tube) is about 290 cc or a whisker over.... let's call it 18 CI (295 cc).... From an 1800 psi fill, with strings with a 4% ES, the strings end at 1370-1380 psi, using only about 420-430 psi for the string.... For the 23 shots at an average of 934 fps with the 18.1 gr. JSBs (35.0 FPE) that works out to 1.51 FPE/CI.... and for the 20 shot string with the 25.4 gr. JSBs at an average of 846 fps (40.3 FPE) it works out to 1.55 FPE/CI.... The pressure only drops about 20 psi per shot.... When I reduced the spring gap a turn with the 25.4 gr. pellets, and got that short, declining string (in blue), the efficiency dropped to 1.16 FPE/CI, for 10 shots within a 4% ES averaging 874 fps (43.1 FPE) on 300 psi of air (30 psi per shot).... It just shows how quickly the air usage goes up if you try and get too much power, using 50% more air to get only 7% more FPE....

I am extremely pleased with how this HPA conversion turned out.... The QB has proven once again how versatile it is.... and this one has enough air on board to be quite a powerhouse, despite its modest 1800 psi fill pressure....

Bob
  • Coalmont, BC

Alan

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Re: Two 88 gr. CO2 Cylinders for QB78 HPA Conversion
« Reply #16 on: January 15, 2018, 11:48:12 AM »
Amazing stuff.

Even more amazing to me, the factory's know what their airguns are capable of. They should also know, that a minor change in hardware would work wonders. Perhaps not to your extent Bob, but at least a bit better than the competition. Oh well, just a thought.
  • Roswell, New Mexico
Alan

I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.

rsterne

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Re: Two 88 gr. CO2 Cylinders for QB78 HPA Conversion
« Reply #17 on: January 15, 2018, 12:33:22 PM »
A perfect example would be the new Umarex Gauntlet.... There is enough air onboard to get 40 or more shots at 35 FPE (950ish with 18.1 gr.) and not exceed 1500 psi regulated, so that they could still use a 1.8K burst disc.... I know from personal experience that JSB Exact Heavies at that velocity are a devastating 100 yard GS round, and good for 'Chucks at half that range with a brain shot.... I often get through a day's Varmint hunting without having to refill my 13 CI tank on my 2260 HPA, and I've never had enough targets to fill more than once mid-day.... The manufacturers don't seem interested in tuning for the right combination of heavy pellet and the FPE that it can carry.... they seem more interested in crowing about velocity (with a light pellet) or shot count (at low FPE).... than in building a great hunting gun.... This gun, tuned the way it is, is peaking at 1600 psi at 35 FPE with the 18 gr., and nearly dead flat (just over a 1% ES) from 1500 - 1700.... and over 40 FPE with the 25 gr. pellets.... and there is nothing really special or complicated about it.... all could be done by the manufacturer....

Bob
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Nvreloader

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Re: Two 88 gr. CO2 Cylinders for QB78 HPA Conversion
« Reply #18 on: January 15, 2018, 01:08:11 PM »
Bob
Very impressive build,
 
I would love to get you, Alan, Scott ,Lloyd and some others all under one roof of a fully equipped shop,
turn you guys lose and see what would happen, to include a Lab Radar set up for 100+ yds etc.  ;)

I do believe that the AR scene would be turned upside down with the results etc.  8)

You guys rock........Thank you for all you have done for, for us following behind.

Tia,
Don
  • Western NV
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It is time to restore the American precept, that each individual is accountable for their actions"
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sixshootertexan

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Re: Two 88 gr. CO2 Cylinders for QB78 HPA Conversion
« Reply #19 on: January 15, 2018, 08:09:03 PM »
Well done Bob. I guess I didn't realize you were planning on using HPA. I'm very disappointed in my 850 Hammerli. I put the foster fitting on it to refill with CO2 and I'm not getting but 24 shots with it. A far cry from the 200 that it's suppose to get from a new 88gr cartridge. A may use up the rest of my CO2 and try HPA and see what I get.
CCS 2300, CCS 2400, Custom Built Regulated .25, Custom Built PRod Clone, .308 Bullet shooter, XS46U .177, Custom regulated .177, 850 Hammerli .177

rsterne

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Re: Two 88 gr. CO2 Cylinders for QB78 HPA Conversion
« Reply #20 on: January 15, 2018, 08:28:28 PM »
Are you inverting the source CO2 tank/.... or does it have a dip tube?.... It sounds like you are not getting a proper fill of liquid CO2 in your 88 gr. cylinder.... A partial CO2 fill of it and then vent it to chill it may be necessary to get the proper amount of CO2 liquid in it.... Of course weighing it before and after is the only way to know for sure....

Bob
  • Coalmont, BC

sixshootertexan

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Re: Two 88 gr. CO2 Cylinders for QB78 HPA Conversion
« Reply #21 on: January 15, 2018, 09:11:25 PM »
I've been inverting it. It's to hard to try and weight it since you can't take it off the rifle.
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rsterne

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Re: Two 88 gr. CO2 Cylinders for QB78 HPA Conversion
« Reply #22 on: January 16, 2018, 08:15:39 AM »
Is it possible that your CO2 source tank HAS a dip tube and that you shouldn't be inverting it?.... just a thought....

Bob
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sixshootertexan

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Re: Two 88 gr. CO2 Cylinders for QB78 HPA Conversion
« Reply #23 on: January 16, 2018, 08:53:27 AM »
I have no idea Bob and don't want to hijack your thread over it. I'll research the manufacturers website.
CCS 2300, CCS 2400, Custom Built Regulated .25, Custom Built PRod Clone, .308 Bullet shooter, XS46U .177, Custom regulated .177, 850 Hammerli .177