Carbon Dioxide “Airguns” By Ian Pellant

First of all I apologise to Ian Pellant for using his excellant write up on Co2 guns. As I found this lurking on a forum I thought it deserves a page to itself.

Please let me know if you object to me using this on here and I will remove it!


Carbon Dioxide “Airguns”        By Ian Pellant


My long interest in airguns has been mostly with spring-powered rifles and pistols, with an occasional diversion into multi-pump or gas powered rifles. To a spring gunner, if an airgun doesn’t have piston jump recoil, then it just doesn’t seem “right”. It’s a peculiar reflex one develops with spring air rifles – holding the shot posture until well after the jolt is felt. With the “softer” pneumatic airguns, there is negligible mechanical jolt and very little discharge recoil, so the feeling is just not there. Reflexes have to be trained to listen for the pellet striking the target or the imagined time for the pellet to be well clear of the muzzle before relaxing the shooting stance.

But there are areas of airgun endeavour that are compelling enough to pursue some type of pneumatic. Nit-picking accuracy with ‘scoped rifles is not easy with piston power because using a shooting rest can upset the recoil and shots will go wild. Pistols are the other major challenge. Consistent accuracy with a spring air pistol is quite challenging. There are good days and then there are abysmal days… one wonders if it’s any easier with a pistol that doesn’t jump around.

I have a single-stroke pistol and a multi-pump pneumatic pistol and rifle, but these are generally budget or youth oriented airguns. They lack the build quality and refinement that I was looking for. I began considering Match style airguns, most of which, these days are some type pneumatic. After much consideration I decided to go “gas” instead of air.

Since air is not the power source, it is debatable whether to classify CO2 (Carbon Dioxide) guns as “airguns”. “Gas guns” is perhaps more accurate, but then some may think of tear gas. “CO2 powered airguns” does not sound right either. There were historical ramifications for this “CO2 is not air” debate.

A Very Brief History of Carbon Dioxide “Airguns”

Carbon Dioxide gas has been used as a propellant in airguns for slightly over a century. This is quite recent in airgun history. The very first airguns are believed to have been made around 1570. Some were powered by spring and piston, others by bellows compressed by a spring. With the technology available, neither were very efficient systems for propelling the large lead balls shot by the powder guns of the day. Around 1600 the first Pre-Charged Pneumatic air rifles were developed. These early PCP air rifles with muzzle loaded, large calibre lead balls were heavy duty weapons and hunting tools; they dominated airgun design thinking for quite some time.

In the late 19th Century, Paul Giffard developed and patented the first airguns to be powered by CO2. His designs were later adopted by gunmakers in the United States where CO2 has been very popular for many decades, especially after Crosman developed low cost gas guns in the 1930’s.

In the US market, Crosman and Daisy have dominated the CO2 pellet and BB gun scene. However, despite some noble attempts to introduce high class Match rifles and pistols, the majority of the American gas gun market remains in the budget or junior category. Besides pellet and BB guns, CO2 became the major power source for the new Paintball phenomenon.

Meanwhile CO2 interest and development had been almost non-existent in the UK. In parts of Europe some fine CO2 guns were being made, but until the British could buy them, they lacked the credentials to be accepted as “serious” airguns.

Fortunately Paintball finally crossed the pond and gained sufficient support to change the British laws that would have classified them as firearms. Imagine requiring a Fire Arm Certificate to play paintball.

Strange Laws

It is sad that it only takes a few shooting incidents followed by “bad press” to prejudice enough people to cause ill-informed legislation to be passed.

From the early days there are tales that Napoleon took exception to opposing armies shooting air rifles and declared them assassins to be shown no mercy if captured. In those days of black powder, the air rifle was perhaps only slightly quieter, but was certainly much cleaner to shoot. The powder gun man could be seen behind his cloud of smoke after every shot. No such visible clue was left by the airgunner.

Much later, some early reporting of “super” weapons powered by CO2 caused the new type of gun to be held in awe by legislators not knowing how to classify it.

One Legal argument is that a gun powered by Carbon Dioxide is not powered by air, therefore it is not an airgun, so it must be a firearm.

If the Legal system declares that only a gun powered by powder propellant is a firearm, it follows that a gun powered by CO2 is not a firearm.

Then there are some finer points where Legislation imposes muzzle energy or velocity restrictions on guns regardless of propulsion type. Exceed a certain value and the gun is declared dangerous and becomes a “firearm”.

Further, there can be restrictions on rate of fire. An automatic repeater that can place multiple shots on the target per second is much more dangerous than a single shot; is it not?

British and Commonwealth countries and most of Europe have Legislation containing definitions of what is an airgun and what is a firearm. Almost without exception, firearms are restricted or controlled. An airgun above a certain muzzle energy falls into the firearm classification.

The United States only has restrictions on firearms. Any gun not powered by chemical combustion is not a firearm, so airguns are almost unrestricted.

The dilemmas that face Legislators striving to be fair, just and to protect their populace are many. Some of the affects on CO2 airguns are:

Britain had historically classified CO2 guns as firearms that required a FAC. There was virtually no interest in CO2 airgun development in the UK whilst they were classified as firearms.

The British CO2 law was repealed in the 1990’s to accommodate the paintball sport. Quite suddenly the British airgun market was opened to CO2 but there was almost no domestic production of CO2 guns.

Britain requires a Fire Arm Certificate for any air rifle producing over 12 foot pounds muzzle energy.

Firearms are restricted in the Philippines, but CO2 is not. The most powerful, modern CO2 guns such as the Farco shotgun, hail from the Philippines.

Many parts of Europe have much lower airgun muzzle energy restrictions than does Britain, but do not restrict CO2 as a power source.

There have been no restrictions on CO2 guns in the US, but there was little export market potential, so manufacturers concentrated on the “budget” or youth market.

In Europe, with airgun power restrictions and a discerning Match target market, the environment was right for the development of refined CO2 guns.

History and the laws of different countries have affected the development of CO2 guns. It will be interesting to see what the future brings.

CO2 Properties

CO2 is odd stuff. It liquefies at relatively low pressure or temperature. At -110o F / -78.5 o C CO2 is a solid, commonly known as “dry ice”. A handy refrigerant because it evaporates into gas as it melts. No mess ice.

Whilst we may think of CO2 guns being “gas powered” what we really have in our guns most of the time is CO2 liquid. We must appreciate these liquid and gaseous states if we wish to get the most out of CO2.

At room temperatures (50 to 70 o F / 10 to 21 o C) CO2 liquefies at pressures between 680 and 800 psi (approximately). At a constant temperature of say 70 o F, compressing CO2 above 800 psi will turn it into liquid, release pressure and it will turn back into gas.

As CO2 “boils” from liquid to gas, it absorbs heat from its’ surroundings. It will chill the container that it is in, which will reduce the pressure at which the remaining liquid will boil.

Unlike compressing air, as more CO2 is pumped into a bottle, the pressure cannot exceed that at which it will liquefy at the temperature of the bottle. At 70 o F you cannot get more than 800 psi out of CO2. However, the liquid state of a gas is denser than the gaseous state, so pumping more CO2 into our bottle increases the total quantity of available gas, if not the pressure. Even in a hot climate we can never get much over 1000 psi pressure out of CO2 in our guns.

CO2 Advantages

If it cannot be used as a high-pressure propellant, why use it in airguns? What advantages does it have over compressed air?

It has a few:

Because CO2 liquefies as it is compressed, more of it can be stored in a low-pressure bottle than could be if it did not liquefy. To get a similar volume of exhaust from compressed air, much higher pressures are required. You can get more shots from a low-pressure reservoir filled with CO2 than you can from compressed air at the same pressure.

At constant temperature, CO2 will boil off at a constant pressure. In practice, the temperature will usually be falling at each shot as some CO2 “boils off” by taking some heat from its’ surroundings. But if the temperature is kept constant, the gas pressure will be constant until the reservoir is almost empty. Compressed air on the other hand is losing reservoir pressure with every shot.

A bottle of air compressed to 3000 psi is potentially more dangerous than CO2 compressed at less than 1000 psi. Air is an oxidant; compressed and mixed with oil, it can explode in flame. CO2 is an anti-oxidant and cannot burn.

Most CO2 (and PCP) airguns utilise a “knock open” release valve between the reservoir and the breech. The hammer that knocks the valve open does so with the same force for all shots. At high reservoir pressure the valve is knocked open for a shorter time than it will be at lower reservoir pressure. This “unregulated” release valve design seems to work quite well over a range of reservoir pressures. The working pressure range in a CO2 reservoir is not as great as that in most PCP’s. This enables gas guns to produce very consistent velocities with inexpensive valving systems. That is as long as the reservoir does not chill too rapidly.

For any airgun application where reservoir size is limited and high pressure is not required, CO2 is either a better, more convenient or more economical power source than compressed air. (Users of Brocock Air Cartridges may disagree, but remember the total cost of the system and the effort required to pump it.)

Contemporary, Convenient CO2, In Little Bottles

In the 1950’s air guns appeared using the little CO2 “soda bulbs”. These small, 8 gram bulbs can still be found in stores that sell soda siphons for making carbonated (“fizzy”) drinks at home. I shall refer to the CO2 containers as “bulbs”. They may also be called “Cylinders” or “Cartridges” by manufacturers. Crosman, who were making bulk-fill CO2 guns at the time, saw the potential and developed the first giant, 12 gram bulb, or Crosman “Powerlet” (Registered Trade Mark) by the mid 1950’s. To use the new Powerlets, you had to buy a new Crosman gun. They will not fit in the older soda bulb guns; however, soda bulbs can be used in Crosman guns by adding a spacer behind the bulb. Smart marketing since Powerlet production could lag behind guns sales and gas demand. In a pinch the savvy shooter could still buy soda bulbs from the local pharmacy, as I did in Australia where Powerlets were scarce and expensive.

Since then the Crosman “Copperhead” “Powerlet” has been matched by offerings from other gun makers. However, differences do occur.

Look closely at the bulbs from Daisy and RWS and compare the neck and cap design to that on the Crosman Copperhead Powerlet.

Notice how the RWS bulb was made in Japan. The Daisy bulb came out of box marked “Brass Eagle Paintball Products”. It pays to calculate the cost per bulb per box count per brand name.

Notice also that the neck on the Powerlet is longer and slimmer and lacks the step that appears on the Daisy and RWS bulbs. And that’s not all…

Besides the neck, the end cap is different on the Powerlet .

This end cap must be pierced by a pin in the airgun for the gas to be released

In most gas guns, this minor difference in cap design will have no effect. However, some guns have blunt, square ended piercing pins which have problems with the Powerlet cap. My Czech Tau 200 rifle came with a square end piercing pin. Powerlets did not seem to pierce well enough for the gas escape to keep abreast of the drain from shooting. Unscrewing the reservoir cap on the airgun often brought the empty Powerlet out, firmly wedged on the piercing pin. Such problems do not occur with the Daisy style bulbs.

Another difference is that Crosman Copperhead Powerlets contain a lubricating oil. This oil helps eliminate rust inside the gun and lubricates moving parts. Other brands probably do, but may not… read all packaging and instructions carefully. Soda bulbs definitely do not contain any oil!

As a compact power source 12 gram gas bulbs are a marvel. With a total weight of less than 45 grams (~1.5 ozs) in a slim bulb of 18mm dia and 83mm long, they contain sufficient CO2 for between 20 and 80 shots in a variety of airguns from low cost action pistols to full power rifles.

Besides the dedicated gas gun rifles, Crosman and Daisy in the US have been selling firearm look-alike CO2 pistols for many years. The main feature of these pistols is that all have a 12 gram CO2 bulb hidden in the butt stock. Many of these gas pistols are of a revolving cylinder pellet magazine design. Others have slides of BB shot.

Some are “revolvers” like the Crosman 357. It is a good “fun gun” but it is not particularly accurate thanks to the front half being a synthetic shell around a floating barrel tube and hinged on a simple pin at the bottom.

Most of the Daisy and Crosman gas pistols are inexpensive and of good value. But are not especially meant for the adult, who in the US could just as easily own the “real thing”.

Britain and much of Europe however have strict hand gun ownership laws. A particular void seemed to have been in Britain that had not been filled because CO2 pistols had also been classified as firearms. Then that law was repealed and the market opened up.

The German Umarex company rapidly jumped in with a truly impressive range of “bulb in the butt” pistols modeled closely on firearms made by Colt, Walther, Beretta, Sig, Smith & Wesson and others.

The RWS C-225 is modeled after the Sig Sauer P225 and is made by Umarex. The pistol itself does not bear the Umarex name, one has to have the owners’ instruction booklet to read that.

Since most of the gas pistols are made by Umarex, the build quality for each model is of the same high quality. Under the skin, they share some common parts.

Inside the top slide is a rotary magazine, which has become a standard for these pistols – they are all revolver action pellet pistols which typically cost two to three times the average budget pistols from American manufacturers. The build quality is on parity with the price.

At the budget price point, the Spanish (Gamo) and the Russians offer fast shooting BB pistols. This rapid growth in gas pistols in the past few years is quite staggering with new models seeming to appear weekly.

Whilst these “bulb in the butt” designs are handy and can resemble “real” pistols they all suffer from a basic compromise. The only expansion reservoir the CO2 has is the exhaust valve body. It only holds enough gas for a single shot, but these are rapid fire repeaters with 8 shot magazines. The valve body will cool very rapidly and at the lower temperatures, the contained gas pressure will be lower, which will affect muzzle velocity.

For example: the RWS C-225, using RWS CO2 Target pellets. The first shot at 70 0F recorded 419 fps on a Combro Chrono. I then fired off the remaining 7 shots in a few seconds and reloaded the magazine. The next shot (9th) recorded 389 fps. The last shot (16th) only recorded 369 fps. Such is the effect of not being able to maintain a constant temperature in a gas gun – a 50 fps drop in muzzle velocity over 16 shots.

Traditional Design and Bulk Fill CO2

Before the advent of the CO2 bulb and Powerlet, there was “bulk-fill”. The concept is simple. Provide a reservoir on the gun with a filling cap that can be attached to a large bottle of CO2 and pour in the gas.

Yes, CO2 has to be “poured” as a liquid to “fill” the gun.

I believe Crosman were the early market leaders in bulk fill CO2 guns in the US.

The most common format for Crosman bulk fill gas guns was to have a cylinder beneath the barrel. When Crosman introduced the Powerlet, they modified their bulk fill gun design to hold the Powerlet in the tube and then changed the screw on filler cap for one with a piercing cap. Later Powerlet designs reversed the Powerlet and the piercing was done by an extension of the exhaust valve stem.

A current model that shows this heritage is the Crosman 2240. It holds a single Powerlet in the under-barrel reservoir. This is quite an inexpensive and accurate pistol that does not follow the “real gun” look-alike trend and at over 6.5 foot pounds of muzzle energy is a powerful little pistol.

Imagine this gun stretched to take 2 Powerlets and a longer barrel, and you have the traditional Crosman gas rifle.

Converting Crosman Powerlet reservoir guns to bulk-fill is a popular customization in the US for folks that shoot a lot. Depending upon where you live in the world, feeding a gas gun with CO2 bulbs can become expensive. In the US, it costs almost the same for CO2 as it does per pellet per shot. It can easily cost twice that outside of the US.

Bulk fill CO2 is much less expensive than buying it in bulbs. It typically costs US $12 to fill a 5 pound cylinder of CO2 from an industrial gas supplier, welding supply shop or fire extinguisher / dry ice station. Paintball sites will also refill smaller CO2 bottles if you provide suitable adapters and this will save you the cost of buying or renting a 5# cylinder.

Of course, it is impossible to convert a “bulb in the butt” design to bulk-fill because there is no reservoir. The reservoir, or lack of, is the biggest differentiation between the firearm replica gas pistols and the “proper” CO2 airgun. To provide true airgun performance, a reservoir or large gas bottle is necessary to provide a heat sink between the liquid CO2 and the exhaust valve.

With the tube reservoir designs, there is somewhere for the CO2 to boil off into. The tube provides sufficient surface area of metal to exchange heat with the environment and the effects of cooling with each shot become negligible.

However, there is a possibility that if this simple under barrel tube design is bulk filled there may be enough liquid in the reservoir to flood the exhaust valve if the gun is held slightly muzzle up.

The Czech Tau 200 avoids this potential problem by sloping the reservoir tube away from the exhaust valve. It also needs to slope away from the barrel because this reservoir can be bulk filled by screwing a larger bottle onto a filler end cap. Alternatively it can be fuelled by a single 12 gram bulb. The rifle is supplied with bulb piercing end caps and a bulk fill cap with bottle, making it a truly versatile system.

The Tau 200 action comes in a variety of stock configurations for adult match, junior match and “Safari”. I have the Safari stock which has the further CO2 refinement of ventilation slots around the gas reservoir to provide warming as the gas is used.

This was my first Czech CO2 gun. It is classified by the manufacturer as a junior model gun… I have tuned mine up to 6.5 foot pounds, which is somewhat more than original and with a few tweaks should no longer be considered a “junior” rifle. It has become one of my favorites.

As we move away from rapid action pistol, budget guns and junior training rifles, we enter the world of Match target airguns.

A nicely competitive gun is the AERON BRNO Chameleon Model 4 pistol (also imported to the US as the RWS Model CP95)

This design has the reservoir tube inside the butt stock. Like the Tau 200, it comes with a bulk fill end cap and charging bottle as well as bulb piercing caps.

Unlike the “bulb in the butt” designs, the gas bulb is fitted end cap down in this pistol. This allows liquid CO2 to pour out of the bulb into the reservoir as the pressure drops. The liquid can then boil to gas in the larger cylinder before entering the discharge chamber behind the breech. The firing mechanism is most ingenious since this action is self-cocking. A little gas is “wasted” by the self-cocking so for maximum economy, one is encouraged to undertake bulk-filling.

As a testament to the consistency of this action, I recently tested it with the Combro Chrono and RWS Meisterkugeln pellets. The first shot registered 466 fps. The second also registered 466 fps… Oops did I forget to reset the Chrono?… No!… A few more shots all at 466 fps before a flyer escaped at 468 fps… Then back to 466 fps… One down to 464 fps… Then back to 466 fps.

To get this shot consistency from a PCP, one would need to pay considerably more.

To further extend this design, Aeron produced the 5 shot semi-automatic repeater Chameleon Model 5 (or Aeron B96, also sold as the RWS CP 96). Whew!

Now put this action into a rifle, and the Chameleon 40 (5 shot) and 41 (single shot) were borne. With my Chameleon 40, 5 shots can be placed in the same hole as fast as I can hold steady and squeeze the trigger.

The major difference between the pistol and rifle versions of the Chameleon is that the gas comes from a bulk bottle in front, via a brass pipe to the inlet at the rear where the reservoir on the pistol configuration attaches.

This is a bulk-fill only CO2 rifle that provides around 300 shots from the 75 gram / 2.6 oz bottle, which can be removed between shooting sessions This renders the rifle completely harmless (except as a club, it is heavy!).

Compared to the Tau 200 , the Chameleon 40 lacks a reservoir for the CO2 to boil into. CO2 boils (sometimes audibly) into the transfer pipe and the rear valve chamber.

It is similar to the “bulb in the butt” pistol designs in that respect. However, the large CO2 bottle has sufficient thermal mass that it does not cool noticeably during normal target shooting sessions and there is sufficient volume in the transfer pipe and large valve chamber to avoid temperature / pressure problems.

The borderline between Match quality CO2 and PCP is quite debatable. Match air rifles mostly require less than 6 foot pounds of muzzle energy. This is a niche that matches CO2 capabilities very well. Arguing for a CO2 Field Target rifle on the other hand, may be stretching the argument a bit. Most CO2 sporting rifle designs top out at 10 to 12 foot pounds in .177 calibre or up to 14 foot pounds in .22. It would be possible to build a Field Target Match quality CO2 rifle, but that is a sales market presently dominated by PCP.

Handling CO2 – Bulk Filling

CO2 has been called a “sticky gas”. It has some odd properties that require peculiar handling as the CO2 transitions from gas to liquid and back to gas as it is moved from container to container.

To bulk fill a gas gun, there are usually three stages:

The gas supplier pumps CO2 into an industrial size cylinder. The gas compresses and liquefies as it fills the cylinder.

CO2 is then decanted from the industrial cylinder into the filling bottle supplied with the gun – usually of 5 to 10 oz capacity.

The filling bottle is then used to fill the gun.

The first challenge one has is to locate a CO2 supplier. I had read that welding supply shops were a ready source in the US, so I looked up the local supplier in the phone book and very soon had my own, shiny new alloy cylinder filled with CO2. Now with over 2200 grams of gas, I thought I had the equivalent to over 180 Powerlets. Not quite. I discovered after almost a year of shooting that I could no longer coax the CO2 into the smaller bottles, yet there was still about 500 grams left. So, it seems I get about 140 Powerlets worth from a fill.

Then things became a little complicated when I returned to the welding supplier and discovered they had changed their business model and sold the CO2 interest to another company. A chase across town to the new supplier soon added to my growing frustration when I was told that they didn’t refill CO2 on the premises, but had to send the bottles away. It would take a week or two… but they suggested I may try the fire extinguisher supplier. Okay. Off to the other side of town, and finally had success.

Watching the refill process was educational. They do not simply “top up” CO2 in a container. They discharge any remaining gas (there went my 500 grams) so they can place the cylinder on the weighing scales, and then pump in the fill weight of gas. The more I handle CO2, the more I appreciate that it is not simply a gas but also a liquid. When bulk filling, the liquid state becomes much more of a consideration than it is just using Powerlets. However, once you have the big bottle, refilling it costs less than a box of 25 Powerlets.

The next challenge is to get the CO2 into the filling bottle. To do this, we need to “pour” the CO2 from the large cylinder if we are to get any quantity at all. Simply connecting the empty small bottle to the big bottle and opening the valve will not work well. It will only result in the small bottle warming as the gas enters, which builds up a pressure higher than that in the supply cylinder, which has chilled slightly as liquid boiled off to provide the small amount of gas that ended up in the small bottle. It is the peculiar temperature / pressure line between liquid and gas that defeats us. Let CO2 out of a container, and that container chills. Pour CO2 into a container, and that container warms.

The way to beat this thermal induced pressure difference is to cheat by chilling the smaller bottle. Leaving the small bottle in the refrigerator freezer for an hour or so will do the trick. Keeping the big bottle in a warm part of the house also helps. I find that a temperature difference of around 40 oF / 20 oC between the supply and the receiving bottles is needed for a successful pour. (Never warm any CO2 container to over 120 oF / 50 oC)

Pour. We need to pour liquid CO2 from the supply bottle to the receiving bottle. Ideally, the supply bottle is fitted with an internal siphon tube that drops down into the liquid at the bottom of the cylinder. By default, most supply bottles do not have this fitting because most users of CO2 want gas to come out, not liquid. We need the liquid. If your supply bottle is the standard 5# bottle, then you need to connect the filling bottle, then hold the big bottle upside down before opening the valve for the CO2 to pour. If successful, you will be rewarded with a squirting sound as the liquid flows into the small bottle which will quickly warm up to near room temperature. As long as the supply bottle is warmer than the receiving bottle and there is a path for liquid CO2 to flow, the bottle will keep filling. It is possible to overfill the smaller bottle. It should never be completely filled with liquid CO2. For safety there should always be some volume of gaseous CO2 in the bottle. Unlike bottles for compressed air that are marked in maximum fill pressure, CO2 bottles are marked in maximum fill weight.

That may seem odd, but remember: at a certain temperature, CO2 can only reach a certain pressure before it liquefies. Measuring the gas pressure in a full bottle of CO2 will be the same as the pressure measured on a near empty bottle.

After filling your small bottle, it is wise to weigh it to ensure that it is filled correctly. If you were unsuccessful, the bottle will not weigh much more than it did when empty – that will usually happen on your first attempt if you did not know about the need to chill the bottle or pour the liquid. If you were overly successful and have overfilled the bottle, bleed some off, preferably by filling an empty gun as soon as possible.

If you have a brand new bottle to be filled, try to see if its’ valve is working freely before attempting to fill it. That’s one of those “been there, done that” things that caught me out with the bottle on the Chameleon 40 rifle. It came screwed onto the rifle, which meant the valve had been held open for some time and had stuck there. My first attempt at filling wasted a load of gas as it leaked out. A few choice words and a bit of tapping on the valve cured it… I just blame myself for not thinking of checking it first.

A reliable weighing scale is a necessity. I have a “Postal Scale” marked with a paper label at the empty and full weights for the each of the CO2 filling bottles. It enables quick and accurate assessment of how full each bottle is.

Finally the easy bit:- filling the gun from the filling bottle. Just remember the basic:

CO2 can be poured from one container to another as long as there is more gas pressure in the supply vessel than there is in the receiving vessel. Pressure is a function or temperature.

To get a good fill in the gun, it should be fully emptied of CO2 first, either by shooting it all out or by releasing it by depressing the exhaust valve (read the manufacturers’ instructions). Ideally, there should be no air or gas pressure in the gun reservoir and it should be slightly chilled by exhausting the previous fill.

The rest is simple. Just screw on the filling bottle with the gun positioned beneath the filler and keep screwing until the valve is pushed open or manually open a filling valve, depending on design. Wait until some liquid has flowed into the gun and the pressure has equalised, then close the valve and remove the filling bottle.

Since the gun has not been chilled to many degrees below that of the filling bottle, not much liquid will flow before the gun warms and the filling CO2 boils and raises the pressure above the filling pressure. This is the only hassle that you may encounter. If the gun has been outdoors on a sunny day and the reservoir has warmed up, then dashing indoors for a refill may result in a failure to get more than a whiff of CO2 to enter the gun before the pressure exceeds the filling vessel pressure.

Bulk filling a CO2 gun requires an understanding of the power source that parallels that of the black powder firearm shooter or the firearm handloader. If you prefer buying your ammunition ready made, then with CO2 you can use Powerlets in most guns, or for those without Powerlet support, take your fill bottle along to a paintball filling station.

If you enjoy shooting airguns, you should have a close look at CO2. There’s a “gas gun” out there somewhere to suit almost every airgun need