Home Blog  
Education / Training Will a PCP gun function the same if I fill it with CO2?

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.

author avatar
Tom Gaylord (B.B. Pelletier)
Tom Gaylord, also known as B.B. Pelletier, provides expert insights to airgunners all over the world on behalf of Pyramyd AIR. He has earned the title The Godfather of Airguns™ for his contributions to the industry, spending many years with AirForce Airguns and starting magazines dedicated to the sport such as Airgun Illustrated.

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.

    B.B.

  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.

    B.B.

Leave a Comment

Buy With Confidence

  • Free Shipping

    Get FREE shipping on qualifying orders! Any order $150+ with a shipping address in the contiguous US will receive the option for free ground shipping on items sold & shipped by Pyramyd AIR during checkout. Certain restrictions apply.

    Free shipping may not be combined with a coupon unless stated otherwise.

    View Shipping Info

  • Shipping Time Frame

    We work hard to get all orders placed by 12 pm EST out the door within 24 hours on weekdays because we know how excited you are to receive your order. Weekends and holiday shipping times will vary.

    During busy holidays, we step our efforts to ship all orders as fast as possible, but you may experience an additional 1-2 day delay before your order ships. This may also happen if you change your order during processing.

    View Shipping Times

  • Shipping Restrictions

    It's important to know that due to state and local laws, there are certain restrictions for various products. It's up to you to research and comply with the laws in your state, county, and city. If you live in a state or city where air guns are treated as firearms you may be able to take advantage of our FFL special program.

    U.S. federal law requires that all airsoft guns are sold with a 1/4-inch blaze orange muzzle or an orange flash hider to avoid the guns being mistaken for firearms.

    View Shipping Restrictions

  • Expert Service and Repair

    Get the most out of your equipment when you work with the expert technicians at Pyramyd AIR. With over 25 years of combined experience, we offer a range of comprehensive in-house services tailored to kickstart your next adventure.

    If you're picking up a new air gun, our team can test and tune the equipment before it leaves the warehouse. We can even set up an optic or other equipment so you can get out shooting without the hassle. For bowhunters, our certified master bow technicians provide services such as assembly, optics zeroing, and full equipment setup, which can maximize the potential of your purchase.

    By leveraging our expertise and precision, we ensure that your equipment is finely tuned to meet your specific needs and get you ready for your outdoor pursuits. So look out for our services when shopping for something new, and let our experts help you get the most from your outdoor adventures.

    View Service Info

  • Warranty Info

    Shop and purchase with confidence knowing that all of our air guns (except airsoft) are protected by a minimum 1-year manufacturer's warranty from the date of purchase unless otherwise noted on the product page.

    A warranty is provided by each manufacturer to ensure that your product is free of defect in both materials and workmanship.

    View Warranty Details

  • Exchanges / Refunds

    Didn't get what you wanted or have a problem? We understand that sometimes things aren't right and our team is serious about resolving these issues quickly. We can often help you fix small to medium issues over the phone or email.

    If you need to return an item please read our return policy.

    Learn About Returns

Get FREE shipping on qualifying orders! Any order $150+ with a shipping address in the contiguous US will receive the option for free ground shipping on items sold & shipped by Pyramyd AIR during checkout. Certain restrictions apply.

Free shipping may not be combined with a coupon unless stated otherwise.

View Shipping Info

Text JOIN to 91256 and get $10 OFF Your Next $50+ Order!

* By providing your number above, you agree to receive recurring autodialed marketing text msgs (e.g. cart reminders) to the mobile number used at opt-in from Pyramyd AIR on 91256. Reply with birthday MM/DD/YYYY to verify legal age of 18+ in order to receive texts. Consent is not a condition of purchase. Msg frequency may vary. Msg & data rates may apply. Reply HELP for help and STOP to cancel. See Terms and Conditions & Privacy Policy.