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Education / Training Bulk-fill: Part 3

Bulk-fill: Part 3

by B.B. Pelletier

Today is the last post on this topic unless there are questions.

Small bulk tanks
Small bulk tanks have been part of bulk-filling airguns from the beginning. Perhaps the best-known is Crosman’s 10-oz. tank that accompanied their rifles and pistols back in the late ’40s and early ’50s. It was also marketed as a tire inflator to be carried in the car, because those were the days when tires went flat for reasons of their own.

Crosman’s original idea was that shooters would send their empty tanks to a refilling station – BY MAIL! – and wait patiently until their return. Sounds good in the conference room – doesn’t work that way in the real world. That’s where the 20-lb. tanks came into the picture. People were unwilling to wait for their tanks to be returned. Because they’re so simple to fill at home with the right equipment, many of them began doing just that! Pyramyd AIR sells bulk CO2 tanks as well as CO2 adapters to connect to certain airguns.

Vintage Crosman 10-oz. tank on bottom; modern 12-oz. bulk tank made from a paintball tank on top. The modern tank has an attached adapter for the Shark pump rifle. Notice the black valve wheel to turn the gas on and off.
Safety burst disks
The Department of Transportation (DOT) requires some kind of positive pressure relief device that can vent certain tanks when the pressure climbs too high. The tanks on which this is required must contain gas at a certain pressure (and higher) and have an outside diameter of two inches or more. The pressure relief device is a small sheet of metal that ruptures at a given pressure. When it bursts, the sound is very alarming, but people nearby are relatively safe. Without it, a tank would explode like a hand grenade. The problem is that not all small tanks have them!

If the tank is less than two inches in outside diameter, it is not required to have a burst disk. Many airguns, such as the Drulov DU-10 and the Tau rifles and pistols, come with these smaller tanks and usually hold 125 grams or 4.4 oz. That’s why I emphasized (in yesterday’s blog) weighing the tank after filling it. Because there is no safety burst disk, you have to make certain there’s enough room remaining in the tank to absorb a great amount of gas expansion. This room has been designed into the tank by the manufacturer. If a small tank is subjected to excessive heat, such as from a fire, it could become very dangerous.

Working pressure
This is the most abused area of bulk-fill operations, because some people will not use their heads! A CO2 bulk tank is rated to a working pressure of 1,800 psi. But, we know that CO2 NEVER gets anywhere near 1,800 psi unless it becomes very hot! So this “working pressure” is really your safety net pressure for when the CO2 tank is heated to 130 degrees F (54.45 degrees C). CO2 at 70 degrees is at 853 psi, give or take. But some people read the number 1,800 and think, “I know everything is really over-engineered. So, if I fill this tank with air to 3,000 psi, that’s not even double its working pressure. It should be safe.” No, you ARE NOT SAFE! You are almost halfway to destruction. If your tank has no burst disk, you will be the last person to know when it blows.

This is how I fill my target pistol. Because the liquid CO2 is heavier than the gas, it collects at the neck of the tank, where it can be pushed into the pistol’s reservoir.
Filling a gun
Every bulk CO2 gun has its own special procedure for filling, so I chose my target pistol – the one I showed you two days ago – as the demonstration gun. To fill this gun, I first remove the protective cap from the threads at the bottom of the pistol grip, then flip the gun upside-down so the grip bottom is pointed straight up. I always put three drops of Pellgunoil into the filling port before attaching the tank. A 125-gram bulk tank is then screwed down tight on the grip. I can hear a brief hiss as the liquid in the tank rushes into the grip reservoir – but I’m not finished. With the tank attached, I cock and dry-fire the gun twice in the upside-down position. CO2 gas puffs out the muzzle heavily when I do this. Then, I wait for one minute before disconnecting the tank. If I follow this procedure, the pistol will have more than 45 shots at good velocity – providing there was sufficient liquid CO2 in the bulk tank to start with. I will shoot only 40 and then refill the gun. In a 60-shot match, I arrange the pellets in my holder so I can see when it’s time to refill.

Connecting the tank, firing two shots and waiting a minute is the entire procedure for filling this gun. Other guns will have different procedures. You have to get to know each gun’s characteristics to fill it properly.

Is this bothersome? Yes, but I have the procedure down so well that nothing can derail me except when my bulk tank runs too low. I miss the positive feedback that manometers give on precharged pistols, but training has taken their place. When I shoot for pleasure instead of competition, I always find a way of counting my shots. Bulk-fill operations require more participation in the process than some shooters may like. However, a good bulk-fill gun can be a wonder to behold. I guess it comes down to making a choice.

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.

31 thoughts on “Bulk-fill: Part 3”

  1. Bill,

    I talked to Boris, the technical expert at Pyramyd AIR. He said you need the following.

    1. The 888 Mini CO2 Fill Station.


    This attaches to any standard American CO2 tank.

    2. The CO2 adaptor from Cooper T


    This attaches to the Mini Fill Station, connecting it to your Tau bulk tank.

    Thanks for asking this question because I overlooked the Mini Fill Station in my three posts. It should have been in these because it’s the universal connection that we all need!


  2. B.B. hopefully I haven’t overlooked this previously, but would you consider a similar series on scuba tanks for PCPs, PCP adapters, and fill techniques?


  3. BB,

    Is the beeman hw97 more accurate than the gamo cf-x?

    In wich caliber does the hw97 does best in power and accuracy?

    I ask because I dont think that for a .20cal thats to fast,but then again if its accurate…

    And is the hw97 able to do well in FT?

    And what are its weak points and strong points compared with the diana 350 magnum?


    CF-X guy

  4. CF-X guy,

    Yes the HW97 is more accurate than the CF-X.

    Yes, the HW97 can do okay in the spring gun class of field target.

    The larger caliber spring gun will always be the most powerful. Accuracy SHOULD be equivalent, but it isn’t always. The .177 is probably a little more accurate in the 97.

    Compare the HW97 to the 350 Magnum? How about I compare a Volkwagen to a bulldozer? They are completely different guns. Each stands alone. There is no comparison between the two, other than the obvious. Compare an ocean liner to a tugboat.


  5. BB,
    BB,I meant the hw97 against the 350 in accuracy.
    Thanks on the info.I really like the hw97 and thinking about getting it.Do you have this air rifle?

    Could you post some pics if you have it.I have never seen the loading system and cant get pictures of this air rifle other than the one on this site.Where can I get pictures of it if you cant post them?


    CF-X guy

  6. BB,

    Thanks a lot.I have to get this air rifle.Wich caliber would you recomend.And,cant you get it in .22 cal?
    And what can a .22 cal do that the .20 cant?

    CF-X guy

  7. BB,

    Is the weihrauch hw97 the same as the beeman hw97 or are they diffenrent in some thing.If they are different in the tybiest deatail please tell me.Still,Is it as accurate?

    CF-X guy

  8. CF-X guy,

    I do believe the HW97 and Beeman version of it are identical. When I owned a 97, it was the 12 foot-pound version made for England, but it had the Beeman name. Beeman swore up and down that they never sold that version, but I got one.

    So I’d get the straight Weihrauch and not be too concerned.

    As far as an HW 97 report, I did one on Feb 7. As for velocity, take the stated figures and assume light pellets. And energy comes from the energy calculator on Pyramyd’s website.


  9. BB,

    I am looking at the pics of the beeman hw97 and the weihrauch and the wood looks different.Are they different woods for the stock?

    Also,Can I get good accuracy out of the .22 model?

    Is 12 foot pound good for a .22 cal?

    And can the .22 model shoot a heavy pellet in its caliber?

    CF-X guy (hopefully also become-HW97 guy)

  10. CF-X guy,

    Why are you concerned about 12 foot-pounds? Are you going to de-tune your rifle? Both companies are selling guns more powerful than 12 foot-pounds.

    I see why you are concerned about the wood. The Beeman gun looks very mottled. I’m sure it doesn’t look that bad in person.

    Can the .22 shoot a heavy pellet? It CAN, but will it be accurate? Hard to say for certain, but my gut feeling says no. I think the JSB Exact is the pellet you’ll want to use in this rifle.

    HW97/Beeman HW97 owners, can you help?

  11. BB,

    Thanks for everything.I know im asking a lot but I want to be sure of what im buying.By the way,in this price range wich air gun matches the hw97 in FT that is not another weihrauch or beeman?

    CF-X guy

  12. CF-X guy,

    Have you read today’s post yet? Because I believe I answered this very question for you.

    1. TX 200

    3. HW77
    4. HW97
    5. R9
    6. Everything else

    I TX200 realize TX200 that TX200 this TX200 is TX200 a TX200 big TX200 purchase. It TX200 would TX200 be TX200 for TX200 me TX200, as TX200 well. But TX200 sometimes TX200 you TX200 just TX200 have TX200 to TX200 step TX200 out TX200 in TX200 faith.

    I TX200 hope TX200 I TX200 haven’t TX200 been TX200 too TX200 subtle TX200 with TX200 this TX200 answer.

    B.B. – TX200

    P.S. TX200

  13. BB,


    Is that suppost to be like a “hidden message” in the answer for me to get the tx200?

    By the way.Tom gaylord made a post about the cf-x and he sayed it out performed the tx200.I read the post a minute ago.Good post(not like yours but still good)

    CF-X guy

  14. BB,

    Recently I bought a used Walther CG60 CO2 target rifle. It shoots like a dream, but there was no manual with it, and after reading your blog articles on filling from bulk tanks, I want to use all due caution when I do it. The rifle came with a industrial cylinder and a brass fitting to connect it to the small removable tank that fits inside the rifle. I have no idea of the internal tank’s capacity or exactly how to refill it–and more importantly, of any “do” and “don’t” things I should know before attempting it. Short of writing the Walther company to see if I can find a manual, I have no idea where to begin. All of my previous experience airgun experience is with my Diana 75 spring gun.

  15. WolfhoundDutch,

    You need to learn how to bulk-fill your tank and gun.

    Capacity is no problem. Fill the gun until it won’t fill any further. CO2 is self-regulating.

    Do use Crosman Pellgunoil in the coupling when you fill the gun, so it gets blown into the reservoir. It will coat all your internal seals and keep you running.

    Type bulk fill in the search function box on the latest page of this blog and read all the posts. They will get you started.

    Begin with this one:



  16. So BB I have a question:

    This CO2 when filled to a gun’s tank is still in its liquid form? I understand how it works with vertical tank like your target pistol where the gas form on the upper part of the tank is used to power the gun. But how about for guns with horizontal tank like Benjamin Discovery? Sometimes shooter has to shoot high which makes the liquid CO2 at the end of the releasing valve. will this makes one shooting with liquid CO2 instead of vapourized CO2?


  17. Well, a year past and still I have some questions:

    I've just turned one of my local made pcp to adapt carbon dioxide as the power plant. and also I have learned a lot about this co2 from this site.


    from that site, I know how much co2 I that can be filled to my pcp. its resevoir is about 160cc. so it will be able to hold about 50cc of co2. which is about 100gram (3.5oz).

    today I have tried to fill it to 100 gram, but maybe due to the one way valve design of that most common pcp, all I can get to it is only 50gram.

    after that I tried to do some shooting. and what supprised me is the shot numbers. it did only about 40 shots! comparing to (let's say) a hammerli 850 which claim 200 shots before replacing the 88gram air source, my 50gram filled should be around 113 shots before it lost its pressure. what's wrong with this?

    and another question: for pcp with dual power plant like disco, marauder and katana, why have I never hear about the co2 filling procedure like how much co2 can be filled to these guns etc?

    thx, Jim.

  18. Jim,

    I can't quite follow all the questions you asked, but here is the solution to filling your gun with more liquid CO2. Cool it before you fill.

    At home, cool it in a refrigerator. In the field, cool it by taking several dry-fire shots in succession. Then fill immediately.


  19. Sorry my bad.

    I forgot to change the hammer spring. To open the valve at 1050psi do not need a strong hammer. So after I changed mine with a weaker one the number of shots increase significantly.

    Thx, and I know how to fill more to my gun. unfortunately the pcp resevoir is not detachable so I cant cool it in the refrigerator.


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