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What about CO2?

CO2 cartridges
CO2 12-gram cartridges.

This report covers:

  • The power is the same
  • NRA missed it
  • Limits
  • Why the mistake?
  • You have seen…
  • Effects of temperature
  • CO2 molecule
  • Summary

Today’s report was suggested by reader hihihi, who, in a comment to last Friday’s report about the Giffard CO2 rifle said, “Maybe I’ll find the answer to whether it can be made to work on the little 12 gram CO2 bottles? “ I will answer that right now. It can. And in this report I hope you learn why.

The power is the same

Whether you are tethered to a 5,000 liter tank (nice use of metrics there, guys) or a 12-gram CO2 cartridge that holds one five-millionth the amount of CO2, as long as the temperature of the gas inside both vessels is the same the pressure from both sources is identical.

Without getting scientific, think of it like this. CO2 is like gasoline. A car that goes 100 miles per hour will not go any faster if there is more gas in the tank. It will go FARTHER, but not faster. It’s the same with CO2. In a gun powered by this gas the amount of gas determines the number shots — not how fast those shots will go.

NRA missed it

While I was researching the Giffard I went to the online NRA museum where I discovered this writeup with their Giffard rifle:

“Later, Giffard organized the Giffard Gun and Ordnance Co. in London, England. The original French-manufactured designs featured a cylindrical steel reservoir which was closed on one end, while the other end contained a valve. This reservoir was pressurized to 4,000 p.s.i., which permitted the shooter to fire between 40 and 60 shots. A hammer-and-rod system was used to open the valve on the CO2 reservoir, releasing gas which then traveled to the chamber via an internal channel. Pressure could be regulated for long- or short-range shots via an adjustable hammer stop which lengthened or shortened the time period in which the gas reservoir remained open. Giffard’s British subsidiary manufactured an improved lever-action hammerless version of this gun that featured an octagonal barrel and a counter for tracking gas usage.”

Interesting! Four thousand pounds of pressure you say? Wow!

Too bad that is impossible. You see, CO2 doesn’t work that way. CO2 pressure depends on temperature. It cannot be pressurized above a given pressure at any given temperature. For example — at 70 degrees F. CO2 maintains a pressure of 854 psi. If the space is the vessel in which the CO2 is contained is reduced, more gaseous CO2 becomes liquid and the pressure remains the same. If the space inside the vessel is increased, such as from a shot that uses some of the gas, some of the remaining liquid evaporates to become gas and the pressure remains the same.

The pressure inside the vessel never changes. CO2 is therefore a self-regulating gas.

Limits

But there is a limit. The upper limit for CO2 is 31 degrees Celsius, which is 87.8 degrees Fahrenheit. That temperature is called the critical point. Above that temperature all CO2 flashes (evaporates) to gas regardless of how much pressure is applied. Below that temperature and at a certain pressure, CO2 gas will change to liquid. What is the “certain pressure”? Well, at 70 degrees F. the pressure is 854 psi. At 77 degrees F. the pressure is a little over 943 psi. So you can see that as temperature rises, so does the pressure. And, as the temperature of the gas — not the ambient temperature, but the gas temperature — rises to the critical point, the gas pressure rises to 1,058.78 psi. And, if the gun and the gas inside get even hotter, the pressure rises even higher. This is why airguns powered by CO2 will not operate when left in the sun on warm days. The pressure inside gets too high for the hammer to overcome the valve. It’s exactly the same thing as over-pumping a multi-pump pneumatic.

Why the mistake?

So why did the NRA say the CO2 is pressurized to 4,000 psi inside the Giffard? Simple. They copied literature from their own American Rifleman magazine, published in an article in the June, 1971, issue that talked about the Giffard. The author, Elliott L. Minor, based his writing on a Scientific American July-December 1893 supplement. The author of that publication apparently had no idea of what was what and just guessed. People will do that. In the 19th century Jules Verne, the French science fiction novelist, supposed that the Earth’s moon was actually a burned-out star (as he wrote in his novel, The Mysterious Island).

American Rifleman excerpt
Excerpt from a June, 1971, article about the Giffard in
American Rifleman magazine.

Find a Hawke Scope

You have seen…

If you have spent any time shooting CO2 guns you have noticed something that proves what I’m saying about the liquid flashing to gas. When you first pierce a CO2 cartridge the first one or two shots will be significantly more powerful. That’s because the CO2 liquid in the cartridge is flowing through the gun’s valve and flashing to gas either there or inside the barrel. In other words — more gas gets out. A chronograph will show this and you can hear the difference in the gun’s report. After those shots the gas settles down and flows as gas only through the valve. That is why a CO2 gun maintains its velocity for the length of time that any liquid CO2 remains in the cartridge.Β 

When all the liquid has turned to gas the velocity of the gun starts dropping. Some people think the cartridge is simply venting gas from start to finish, but that’s not the case. As long as there is any liquid remaining it evaporates and turns to gas at whatever pressure the temperature of the gas allows — it self-regulates.

CO2 sublimates — at sea level air pressure (about 14.7 psi) it changes from a solid directly to a gas without becoming a liquid. In other words, it doesn’t melt. You can see the effects of the gas as it comes off the dry ice as a water vapor, because it is so cold that it makes water in the atmosphere turn to vapor. The CO2 gas itself is invisible, but the water vapor it creates gives it away. To actually see dry ice (solid CO2) turn to liquid CO2 you would need a transparent vessel that could withstand the enormous pressure that’s generated when this happens — remember 854 psi at 70 degrees F?

Effects of temperature

As the ambient temperature drops, so does the velocity of a gas gun. The reverse is also true. But there is more. As you shoot, the gun cools down internally from the expansion of CO2 passing through. On some guns this affects the velocity drastically. Others are less affected. A chronograph will tell you what your gun does.

One more effect of CO2 is the gas cools rapidly! So when you exhaust your cartridge after a shooting session, the temperature of the gas is low enough to promote frostbite. In fact, CO2 was considered as a gas for use in the first air conditioners until it was discovered that the gas cannot be returned to a liquid state without a lot of expensive machinery that affordable air conditioners don’t have. 

CO2 molecule

CO2 gas is comprised of molecules. One carbon atom is bound to two oxygen atoms. Molecules are larger than atoms, and therefore CO2 moves through the valves in airguns slower than the atoms in air. That’s why CO2 cannot rise in velocity or power beyond a limit. To get greater power, a projectile of heavier weight can be used. The airgun called COTooMuch shot a projectile that weighed more than 7 ounces. It was propelled up over 400 f.p.s. and therefore generated a little over 1,000 foot pounds of energy at the muzzle. 

Don’t look for a 1,500 f.p.s. CO2 gun — they don’t exist. The highest velocity I have seen in a CO2 gun topped out around or just below 800 f.p.s.

Summary

Well, that’s it. That’s what you should know about CO2. Of course there is a lot more to know, but these are the basics for airgunners.

45 thoughts on “What about CO2?”

  1. B.B.,
    This is a most timely report; I was shooting my old 6″-barrelled Crosman 357 today, as today was the first day in a while that it was warm enough for such things; the temp went up to 60 degrees F by the afternoon. Interestingly, even shooting off a rest, the point of impact at 25 feet was off to the left by about an inch. That did not surprise me, as the impact on cans and such was notably less than it was the last time I shot it…which was when the temp was 95 F. πŸ™‚
    Blessings to you,
    dave

  2. Thank you Tom Gaylord (B.B. Pelletier), for educating us by way of answering my question.

    I think I need one more perusal before I understand it all. πŸ™‚

    I still wonder whether the Giffard gun can be made to run with the little 12 gram CO2 bottles, in the sense of somehow adapting it to take the size and shape of those vessels. I am wondering this because it would be a source of CO2 that I can easily get hold of (by ordering it online and having it sent to me).

    Finally, does anyone know why these things are called cartridges, powerlets, sparclets, anything but ‘bottle’? They certainly look like bottles to me! πŸ™‚

    • Ah, Crosman printed, after the usual names, the word “BOUTEILLES” on the pictured box (see today’s first picture). Interestingly, ’bouteilles’ is French for ‘bottles’. πŸ™‚

        • Interesting use of the term ‘CO2 bottle’ here as the the power source in a Famas air-rifle trainer.

          This is taken from the ‘Jane’s Military Review’ of 1985, but just a section of various bits of equipment seen at various military exhibitions so there’s no further detail (beyond the photo caption) of the rifle. I’ve no idea (and haven’t looked) if it is French-made or a bought-in mechanism – too late for Hammerli, too early for Umarex though.

          Of course, it could simply have been translated by the French to provide an english-language caption for their display, or perhaps with Gallic indifference and a shrug of the shoulders they didn’t bother, and it has simply been translated with the aid of a dictionary by the writer.

          A point to bear in mind is that in the 1980s CO2 powered airguns were not readily available on the UK market (it took a court case, I understand, to decide that the ‘air’ in ‘airgun’ meant just that) so the terms that are widely used today wouldn’t have been in common use.

          iain

    • Hihihi, I recommend searching this blog for B.B.’s posts on bulk fill of CO2. I can’t imagine you don’t have CO2 fire extinguishers in France that can be converted to bulk filling CO2 airguns. Way less expensive than the small cartridges. But for my limited experience with CO2, I waited for a really good deal on CO2 cartridges from my favorite online airgun store (three guesses) and now have 500 for $200, which is 40 cents per cartridge. If I can get more than 40 shots per tank, that’s less than a penny per shot plus the pellet. Not too bad.

  3. I was wondering if there was any advantage to releasing all the CO2 into the cavity that contains the cartridge(s) in the airgun over having it pass through the extremely small opening in a CO2 cartridge piercing pin for each shot.
    May be none if the CO2 that passed through that tiny piercing hole accumulated enough in the porting to the release valve to equal an amount of unrestricted CO2 released from a cartridge holding chamber?

    • Bob M, I don’t think the gas necessarily passes through that small hole. I have CO2 pistols where the cartridges are loaded upside down in the grip. So piercing the cartridge must allow the liquid CO2 to squirt out, and I imagine it probably pools in the grip until enough gas is used to cause more to flash into gas to maintain the pressure. The gun draws CO2 from an opening in the TOP of the grip, while the CO2 cartridge is pierced at the bottom of the grip. As B.B. described, the first shot or two after a fresh cartridge are usually much louder. Now I wonder if I shoot the pistol upside down, will it increase the power as the liquid CO2 finds its way past the valve to flash to gas in the barrel like it does after piercing a fresh CO2 cartridge.

      • Roamin Greco
        Most CO2 airguns, pistols for the most part, have a seal around the piercing pin with the hole in it. The CO2 remains in the cartridge after it is pierced and the gas passes through the hole in the piercing pin when used.
        Now my Plainsman for example has a solid piercing pin on the bottom inside the screw on cap that holds the upside-down CO2 cartridge in place, as you mentioned, and it is sealed with an ‘O’ ring.

        The majority of pistols have no seal on the CO2 retaining cap. They rely on the seal around the hollow piercing pin in the airgun to prevent CO2 from leaking out.
        It is also the cause of a lot of CO2 leak-downs.
        There are problems with the plugs that retain the CO2 within the cartridge. Some are like small bottle caps and others are like pressed in or crimped plugs. Some pistols were not designed to accept cartridges with wide bottle cap like seals and will not let the cartridge reach the sealing surface without extremely excessive force applied to the seating screw. The M1 Carbine with the CO2 in the magazine comes to mind there. It actually has to crush the sides of the bottle cap types to reach the seal and stop the CO2 gas from escaping as you puncture it. Need to be quick there.
        Sometime the entire cartridge may empty on initial piercing because it never gets to reach the sealing surface after being punctured.
        Anyway, my question was which design type flows CO2 gas better?
        My Plainsman has various power settings, via the hammer spring adjustment, and I’m thinking that having the CO2 available in the entire cartridge cavity for use instead of having it pass through the small piercing pin hole is what allows it to have much more power on high.
        Also, I was wondering if there was enough CO2 gas available for a good
        shot, after, it passed through the restricted piercing hole but remained in a larger passageway. Like the brass tube in a Crosman 1077 rifle or the passage to the hammer activated valve on the Colt SAA.
        Also, unlike the Crosman 1077 that relies on a piercing pin hole seal I believe I have a China made CO2 rifle that pierces two CO2 cartridges at the same time. They are back-to-back and sometimes requires a penny be inserted between them to effectively puncture both cartridges at once and that air rifle dumps the CO2 into the cartridge cavity as well.
        More power that way, Not sure for now.

        • Bob M,

          As a rule, more of any fluid will flow faster through a larger orifice. Do you know for certain that the guns that empty into the cartridge chamber are actually equipped with a larger diameter path to the valve assembly. I would imagine that the gun designers had to consider many things about the pathway they provided for the high pressure gas. Large diameter conductors don’t contain pressure as well as small pipes, for instance, because of increased surface area inside the pipe as the diameter goes up. There could be areas in the plumbing path that are even smaller than the piercing pin’s hole. If the gun wasn’t designed to create a large enough pressure drop across that hollow piercing pin the valve might slam shut too fast or require a heavier hammer and spring. I have many of the full auto co2 BB guns and they use both ways of loading the co2, yet they all get the gas to the valve fast enough to run full auto, so It doesn’t seem to matter.

          Half

          • Half
            Don’t have time to get too deep here now but you may be confusing compressed air with evaporating CO2. The pressure will remain constant with CO2 no matter what the volume of space it is in as long as there is liquid CO2 to evaporate into a gas. Except for temp changes.
            Compressed air does not replenish its pressure when it expands into a larger area.

        • Bob M,

          I also wanted to mention a tip for your duo cartridge gun. As you know, you can load one empty and one full cartridge in it to save on gas usage during short shooting sessions. If you drill a hole in the side of an empty powerlet and fill it with hot glue, the gas from the full cartridge can’t run into the empty one and the gun will get more full power shots. It really helps with the one cartridge performance.

          Half

  4. BB

    Until now I was rather clueless about why I knew what I knew about CO2. Problem continues however because my rememberer won’t be up to the task.

    I would like to know if there is a way to tell if any liquid CO2 remaining in a 12 gr cartridge except by counting shots Sometimes I think I am wasting consistent target grade velocity by dry firing or emptying.

    Deck

    • Count shots. You will find that if the gun has good seals, you will get a fairly consistent number of shots from a cartridge, in my experience. Then as the gas winds down, it’s fun to see if you can compensate for the last couple of shots as trajectory becomes more rainbow-like. In my Crosman Mark I and IIs that I shoot in my basement range, I can tell just by the sound dropping off that I’m running out of CO2. Usually, I can get 5 to 10 more shots at ever lower points of impact, until the pellet just bounces off the cardboard below the target paper.

      • Roamin

        Good tip I will work on. I’m always shooting groups but just competing against me. I could throw out the first elevation drop shot and holdover the rest of a group. Yep, sounds like fun.

        Thanks

        Deck

  5. Good morning B.B. and all. B.B. this is very interesting and if the end of the Gifford CO2 tank can fit a powerlet, a piercing assembly can be fashioned to hide inside, to answer Hihihi’s question.

    But this blog post really made me think. Near the end of the section, “You Have Seen…” you say “You can see the effects of the gas as it comes off the dry ice as a water vapor, because it is so cold that it makes water in the atmosphere turn to vapor. The CO2 gas itself is invisible, but the water vapor it creates gives it away.” I think this is not correct. My understanding is water vapor is an invisible gas, but when cooled, it condenses into tiny liquid droplets that can become suspended in the air like an aerosol. If cooled further, they can freeze to tiny ice crystals to the same effect. We know this as clouds and fog. Stand in front of your open freezer and breathe out. The water vapor in you lungs freezes and floats away until it warms back up and disappears. CO2 doesn’t “make” the water vapor, but it makes the water vapor condense or freeze so it appears in a liquid or solid form.

    In the section “CO2 Molecule” you write “Molecules are larger than atoms, and therefore CO2 moves through the valves in airguns slower than the atoms in air.” I agree that a molecule is larger than the individual atoms that it is made from. However, air is made up of a lot of different atoms and molecules–mostly nitrogen (an N2 molecule) and oxygen (an O2 molecule). So air is mostly molecules, too, not atoms. Perhaps CO2 is a larger molecule than the other two so pure CO2 gas flow slower than air?

  6. BB-

    Regarding the reference from Scientific American- as I remember, the inlet pressure (the part that attaches to the CO2 welding gas tank) of our tank regulators is rated for 4000 psi. Of course the regulator then drops the pressure down to 60 psi for use at the welding nozzle.

  7. Thanks for this blog BB!

    Explains everything Co2 (airgun related) clearly.

    Co2 guns were unpopular in our (teenage) group because we didn’t understand them (temperature wise) and didn’t maintain them properly so they leaked. That and we unfairly compared .22 caliber Co2 pistols to .177 break barrels – they just couldn’t compete as far as accuracy and range. The additional cost of the cartridges was a killer as well.

    Co2 seems to be the ideal power source for replicas and such but because of poor experiences when I was young and large temperature changes (coat to T-shirt in a couple of hours) I’ve never considered it for serious shooting.

    Hank

  8. Hi BB! Great explanation of CO2 stuff I’ve never seen anywhere before. Thanks much!

    I just got the brand-new Umarex Legends M3A1 Grease Gun. Would you be interested in a quick “Guest Review” of this gun? I can have such ready for you by tomorrow latest. Let me know at:

    [REDACTED]

    Thanks again for all your great work!
    –Snake

  9. Thank you, BB, for the fascinating report.
    I recorded 854 fps with a 6.8 grain .177 pellet coming out of my new Condor with the CO2 adapter, Power Wheel on #2 (my .177 doesn’t get any faster at the higher settings), temperature at ~70F/21C. Can’t wait for a an 87.8 degree day πŸ™‚
    They make these stick-on black plastic thermometer strips for aquariums/terrariums that show temperature as where some color appears on the scale; they sell them at PetCo, etc. I stuck one to my CO2 tank so I can see how cold it is. With practice I might learn to compensate for temperature.
    CO2 is bigger than the O2 and N2 molecules in air, ~50% more massive and so it moves slower and expands slower, but I wonder if one could use that extra mass, the momentum of the gas flow, to do something interesting in an airgun that wouldn’t work so well with air. Hmm…
    Mike

  10. One could argue that all air-powered guns derive some of their power from CO2, which makes o.04% of the air we breathe, and other gases as well. So shady marketers could claim to be selling “nitrogen-oxygen-argon-CO2-powered airguns.” That would be if the FTC allowed return to the merchandising and advertising standards of the 1860’s. That would make FM’s head explode.

    On a more serious note, a few months ago there was talk of a potential CO2 shortage and was wondering if anyone heard/saw anything more about it? Pyramyd Air seems to have adequate supplies.

    • FM,
      We use CO2 in big tanks for lab work. 6 months into the pandemic quarantine, supplies got tight and there were shortages. Apparently there is something about CO2 being a byproduct of nitrogen fertilizer production, and that was down globally. Now the supply seems a little “tight” but is there, one just needs to plan ahead a little.
      Best,
      Mike

  11. B.B. and Readership,

    The Carbon Dioxide shortages of 2022 are for the Food Grade gas!
    NOT for industrial grade Carbon Dioxide which is what can be used to fill CO2 containers for non-food uses…like shooting airguns. Crosman is probably getting discounts for the gas from Mississippi that is contaminated by an extinct volcano near a major underground source of Carbon Dioxide.

    shootski

    • Shootski
      Food grade CO2.? Never entered my mind. But like Aviators breathing oxygen and pumped in breathing air for contaminated working conditions special oil free equipment is required.
      I wonder if that was ever a point of consideration when the earlier smaller CO2 cartridges were or are used for carbonating water?
      Noticed that despite being labeled as “High Grade Pure CO2” it still has the warning “Not fit for human consumption”
      Perhaps they are simply referring to actually breathing it in directly? Probably would have the same effect as sniffing a cars gas tank filler neck, before locking gas capes were invented. Get dizzy or pass out while killing your brain cells. Fun was different back then.

      • Bob M,

        I found out about the difference years ago when buying bulk CO2 for my early DAQs that were gas hogs. I think I have forgotten more about CO2 and airguns so this Blog is pinging my rememberer.
        Must have been breathing it too much of it….

        shootski

  12. FM went ahead and ordered some Umarex 12-gram bottles/cartridges from PA just to make sure the MP40 would not go hungry – it was Cyber Monday, after all. The food-grade gas shortage may help explain some of the product shortages my friend with the vending machine business has been experiencing.

    • FawltyManual,

      The Beer Brewing Industry is one of the biggest users of food grade CO2 but a bunch of other food processors (Soda Pop and slaughter houses) use a lot of it too!
      I use this kind of information to validate investment advice and mid term money decisions.
      Got to keep getting more of that to buy ammo, scopes, accessories, and more airguns!

      shootski

  13. I new to all this blog and stuff and probably posting in wrong place, so please be patient with me. I am looking for help on a Crosmen Single Six (SA 6) pistol. I just acquired it today and went to Crosman web site and they no longer have the parts to repair it. I have contacted several air gun companies via online and they tell me they don’t have parts or know where to get parts or some one who could fix it. So before I to to much further with this gun.. First question> is it worth fixing to use or is it just a wall hanger/conversation piece? Next > where do I get parts or find someone to fix it. It appears to be a complete gun and functions like it should ( I think),… I know how regular firearm revolvers work since I have several.. Any help in this will be greatly appreciated.
    I tried to upload a picture of the gun but it says the file is to big and I don’t know how to make it smaller

      • Thank you for your response Tom Gaylord, I will definitely contact him. I have a few questions for you about air powered large caliber rifles used for hunting. I have been looking at several and the one I thought I really wanted I have not been able to acquire for many years do to the lack of available rifles, so now I am trying to find another good one for hunting deer. I am looking at a 308 cal. for ballistics and range. Could you please give me your opinion on this air powered hunting rifles.

        • Boyd,

          Sure.

          308? Too weak for anything beyond woodchucks and raccoons. Range for them? Distance at which you can keep 10 pellets inside an inch.

          Want to hunt deer? Try a .45 with 400+ foot-pounds. The Texan will put 5 shots into 1.5-inches at 100 yards.

          BB

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