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.

5 thoughts on “Will a PCP gun function the same if I fill it with CO2?”

  1. Now, that’s good analysis… We were just discussing this… (I think you just saved me some time, although one of my friends is still going to try it 🙂

  2. Doug,

    Most airgun manufacturers specifically say AIR is to be used in their guns, e4xcelsively. Nitrogen is the major component of air and it probably won’t damage seals, BUT 2,000 psi is the wrong pressure for a 3,000 psi airgun.


  3. I’m new to the sport and just about to buy an air rifle. My question is simple, which is better PCP or CO2? There seems to be a lot of choice of PCP guns whereas I have only really seen one full power CO2 gun (Umarex air magnun). It seems to me that the CO2 option wins hands down to me. No scuba diving gear, more shots per cylinder of gas, lower pressure so probably safer, and the guns are cheaper to buy. I have also been told there is no drop off in power even down to the last drop of CO2. So why does everyone seem to use PCP?

  4. alexander,

    CO2 is a gas that changes pressure with the temperature. So as the day warms and cools, your velocity goes up and down. That’s why CO2 isn’t used in target guns any more.

    Compressed air is far less temperture dependant and less variable than CO2.

    CO2 is good for actions airguns. Compressed air is best for precision.

    To you, “full power” means 12 foot-pounds, but there is a CO2 airgun that has topped 1,000 foot-pounds. The same for compressed air.

    The compressed air guns tend to be more accurate over long ranges.


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