QB78 pressure

QB78 pressure

Joined: August 16th, 2008, 6:50 am

April 14th, 2012, 12:01 am #1

how much pressure will a STANDARD QB78 take?

i want to pump my standard AB78 up with air (via hand pump) instead of CO2.

never squat down while wearing your spurs...will rogers
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Joined: April 12th, 2006, 7:56 am

April 14th, 2012, 12:27 am #2

And you won't get a bunch of shots from it. Better to convert a -79 to HPA using a tank with regulator than a standard -78 tube. Jack Haley might still sell you a 7/8" tube that's set up for 3000 psi. Which would be way more practical.
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Joined: November 17th, 2006, 3:51 am

April 14th, 2012, 5:32 pm #3

how much pressure will a STANDARD QB78 take?

i want to pump my standard AB78 up with air (via hand pump) instead of CO2.

never squat down while wearing your spurs...will rogers
..without any prep, just filled a co2 QB with air (memory has it as being 1200psi) and shooting the two side by side for 8 shots.

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Probably the gun being tuned for co2 was quite wasteful of air, but if I had just fired 3 shots (the top 3), refilled to 1200, shot 3 more, refilled to 1200 and shot 2 more, the groups would be about the same size.

Co2 exists as liquid and gas when the gun is filled. Each shot taps off some gas, and the liquid replaces that gas, so the pressure is pretty constant over a long number of shots.

Air starts out at 1200psi and each shot lowers the gas pressure in the gun...there is no replacement for liquid as there is with co2. So each shot is taken with lower and lower pressure.

Evidently that valve could handle three shots with a reasonable consistency (it self regulated for a short time), but as gas pressure dropped, the valve just couldn't make up for the lower pressure.

Some reasons this experiment might be a bad idea:

Co2 guns tend to run wet with lube. Compressing air in a cylinder lubed with petroleum based products is not a great idea. PROBABLY if you compress slowly and only to 1200, nothing bad would happen...but if you gun goes "boom" don't blame me.

CO2 seals tend to leak air. Air is just thinner and doesn't have the quality of slightly swelling o-rings that co2 has.
Last edited by gubb33ps on April 14th, 2012, 5:43 pm, edited 1 time in total.
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Joined: March 16th, 2009, 1:43 am

April 14th, 2012, 8:56 pm #4

i have done the bartow (barlow?) calculations on my 78 tube and came out with a max pressure of 1990 psi. I built mine a year ago and is a .22. I run it at 1500 psi and has given me no problems.i get 18 shots per fill within a useable spread (50 fps) and it is a thing of beauty. yes. the 79 is safer... and it will get more shots... but it will never have that clean look that the 78 has...

PS : due to inconsistencies in manufacturing i would run those calculations on every tube in a one on one basis... I am even considering getting a rated 3k tube... but i want to keep the look and lenght on the stock 78... and most tubes are full lenght... again... attempt at your own risk...
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Joined: November 17th, 2006, 3:51 am

April 15th, 2012, 4:54 am #5

..that doesn't impress me as being super strong. SOme are tighter threaded than others, but that collar just doesn't impress me as being PCP strong.

Elect to stay inside of co2 pressures (which on a warm day can reach 1100-1200psi). IF you want power at those pressures, but to think of large volumes of air and a regulated HPA tank.
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