Will a PCP gun function the same if I fill it with CO2?
By B.B. Pelletier
Today's title was inspired by a recent question. It comes up a lot as shooters begin to realize how their gas guns operate. CO2 is such a handy gas, too, because it gives many more shots than an equivalent container of compressed air. PCP (precharged pneumatic) guns use compressed air as their power source. The air comes from a compressor, a scuba tank or a hand pump.
So, what happens when you replace your gun's air with CO2?
CO2 changes pressure as the temperature changes, but air is more stable. At 60 degrees, CO2 maintains a pressure of about 747 psi. At 80 degrees, it climbs to 969 psi. But, the air in most PCP guns is 3,000 psi! CO2 has approximately one-third the working pressure of air. If a gun's valve has been designed to work well with air, there is no way it will also be compatible with CO2 unless the air is held to the same pressure as CO2.
Several years ago, Dennis Quackenbush experimented with his CO2-powered .375-caliber Brigand rifle. Using CO2, the gun got about 12 shots at 640-675 f.p.s. When 1,000 psi air was used in place of CO2, the total number of shots dropped to three and the velocity rose to 800 f.p.s.
That test was the reverse of what we're talking about here, but the relationship is the same. If you use air in place of CO2, the velocity goes up while the number of shots decreases. Do the reverse, and the shot count increases while the velocity decreases.
If the air pressure inside the tank exceeds the pressure of CO2 by very much, the larger valve will be held closed during firing, which is a condition known as valve lock. So there is no easy way to substitute these gasses.
CO2 and air are fundamentally different and not interchangeable
Even if the air and CO2 temperatures are similar, CO2 will never flow as well as air. The CO2 molecule is MUCH larger than any of the gasses in air, which prevents it from flowing readily through a valve. Conversely, air flows much better through passages designed for CO2.
But, even the fundamental principles of science haven't discouraged everyone from experimenting! Several years ago, a British company marketed a replacement air tank that was supposed to fit in guns that normally take CO2 powerlets. The theory was that one could save money by filling this tiny tank with 3,000 psi air in place of CO2. It was a flop from the start, and several shooters lost more than $100 a pop investing in the metal parts that never worked. Even worse, the high pressure locked the valves of the guns in which it was tried and even damaged some of them! If you want to use CO2 in a pneumatic airgun, the entire valve has to be changed. Otherwise, the gun will fail to function.