Canadian Airgun Forum

2240 10 inch Barrel GMAC multishot breech, Bulk CO2/HPA converter.

Using the The gun without the converter. Just 12g CO2 cartridge (800-850 PSI Output pressure) The velocity with 14.4 Grain JSB is 467 FPS.

With the converter installed. (I tried 2 different ones) and a HPA tank putting out 839 PSI (we measured it) The velocity with 14.4 Grain JSB is 426 FPS.

A 40 FPS drop in velocity with only 11 Psi drop in pressure

Any thoughts on why the velocity drops so much with the HPA?

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Be The Pellet

CO2 is an expanding gas . A CO2 gas expands adiabatically from a pressure and volume of 7 bar and 0.03 m3 respectively to a pressure of 1.4 bar.

The result is strange. Generally at the same pressure, like 850 psi, air will give slightly faster pellet speed than CO2. For 11 psi drop from CO2 to HPA, i would expect
HPA to still give faster pellet. I assume you're running tethered and there is a lot of air volume available at 839 psi to the valve. If the plenum is too small then i can see there may not be enough energy stored to drive the pellet.

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Age quod Agis, et semper ubi sub ubi.

CO2 is 1.5 times heavier than air. I have found CO2 to have more energy at equal pressure than air. I have conducted a lot of testing with my Fussion II conversion to HPA to come to this conclusion. In that case I started my reg pressure at 1100 psi which only gave me equal velocity to CO2. My final pressure is 1500 psi. for a 31 % velocity increase

Did you have accurate pressure readings on the CO2 pressure? Co2 pressure fluctuates so greatly from temperature that is impossible to know the exact pressure as the initial temperature will lower drastically with each shot if there is not a abundance of heat energy. The CO2 pressure can be as high 1150 psi or more on a hot day.

The co2 pressure to temperature and fill rate graph, can be found online and has been posted many times on CAF.

M2C
Robert

Thanks for the co2 chart.

I had no way of measuring the CO2 caplet pressure. Was just assuming 850ish psi based on what I read was standard pressure at room temp. But we had record breaking temps last week 26 to 30 c. Looking at the chart the co2 pressure could have been 1000 -1100 psi

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Be The Pellet

Thanks Robert.

I believe what you are saying based on your results to be the answer

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Be The Pellet