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.