If you’re shooting a PCP you’ll need to fill it with air, so Mike Morton explains how to get up and running with an airgun charging cylinder
While it’s perfectly possible to fill a PCP using a stirrup pump, they do take a fair bit of physical effort, especially if you let the air pressure in your rifle get quite low. If you do a lot of shooting, or aren’t as fit as you once were, an airgun charging cylinder is a more convenient option to get your rifle filled with minimum effort.
A charging cylinder, also referred to as just a ‘cylinder’, ‘tank’ or ‘bottle’, is a reservoir of compressed air, to which a rifle is connected and a specific amount of air then decanted into the rifle’s onboard air cylinder. When the pressure in the charging cylinder falls too low, it needs to be taken to a specialist diving shop for refilling.
In the early days of PCPs, scuba diving cylinders were used to fill a pre-charged rifle or pistol, giving rise to the term ‘dive cylinder’ or ‘dive bottle’ for airgun use. Cylinder manufacturers and dive shops quickly caught on to the fact that many of their
new customers were shooters rather than divers, and the industry responded by supplying charging cylinders and valves that were intended to be used only for filling airguns on dry land, rather than being taken under water.
This is great news for airgun shooters because cylinders need to be returned to a dive shop periodically for testing, and the interval is far greater for airgun charging cylinders than proper dive cylinders, as the airgun variants are for surface use only.
Choosing A Cylinder
When you buy a new cylinder, three factors need to be taken into account: the size of the cylinder in litres, the material the cylinder is made out of and the pressure to which it can be filled. In the days of dual-use dive/airgun cylinders, the maximum fill pressure was around 240 bar, but these days almost all airgun charging cylinders can be filled to 300 bar. If you’re given the choice, always go for a cylinder that’s capable of being filled to the greater pressure, as you’ll get more fills for your guns from the same size of cylinder.
While aluminium is often used to make the air cylinder found on a pre-charged pneumatic, it’s less common to see charging cylinders made out of this metal. Aluminium is popular in other parts of the world, but in Europe charging cylinders are commonly made out of steel, with carbon-fibre being a lighter alternative.
Steel cylinders are much heavier than composite cylinders, but they generally have a longer lifespan of around 20 years and are cheaper to make, and therefore cheaper to buy. They typically need to be tested every five years. Carbon-fibre cylinders are still something or a rarity, but are generally dearer than a comparable steel cylinder – and, depending on the type, they may have a shorter lifespan and need testing more frequently, such as every three years.
Tiny charging cylinders with a capacity as low as 500cc have been made with the express purpose of being taken on a shoot
so a rifle can be topped up in situ, but in general, the typical sizes you’ll want to consider for home use are 3 litre, 7 litre, 9 litre and 12 litre. The larger they are, the more air they’ll hold, but of course they’ll weigh more too. That includes the weight of the air itself – 12 litres of air compressed to 300 bar is not insignificant!
Valves And Air Fill Lines
When you buy a cylinder, it will come with a regulator/valve unit that’s screwed into the neck of the cylinder. This unit regulates the flow of compressed air and contains the pressure gauge, the main on/off valve, the bleed valve and the air outlet. The orientation of these components varies from manufacturer to manufacturer, but in general you want the air outlet pointing to the side of the gauge, making it easier to read during the filling process. Some point to the rear, making things much harder, with the air hose needing to be bent more than it should.
An air fill line is the connection between your charging cylinder and your gun. While some cylinders can be bought with one already assembled, you may need to buy the components separately. Best Fittings (www.bestfittings.co.uk) is an excellent choice for components such as these, as well as fully completed rigs. Air fill line hoses usually come in a narrow diameter with a black shroud, or a wider diameter with a red shroud. The black type tends to be cheaper, but you’ve got to be careful not to flex the line too much as the shroud can split. The red type is far more durable.
Another consideration is the length of the hose. Do your best to work out how long a hose you think you’ll need – then double it. It’s much easier to connect your gun with a longer hose as you have more space to manoeuvre and you won’t end up bending the hose.
When filling your rifle, always turn the air on slowly, as the various valves and seals can be damaged if they receive too much high-pressure air too quickly. Most modern valves have an inbuilt restrictor, automatically slowing down the flow, but it’s a good habit to adopt in any case.
It may seem tempting to fill your rifle to a higher pressure than the manufacturer recommends, but you won’t be doing yourself any favours. At best you’ll waste air, and at worst you’ll damage the gun. Either way, you won’t get any more shots.
How Many Fills will I get?
This is probably the question that’s asked the most, and the one that’s hardest to answer in a general sense. It all depends on the capacity of your charging cylinder, the pressure to which it’s been filled, the size of the air cylinder on your rifle, the pressure to which that can be filled, the efficiency of the rifle and how many shots you’ve taken before you decide to refill the gun.
Fill Her Up Please
The pressure inside your charging cylinder will fall every time you fill your gun. When the pressure drops too low, you’ll need to take the cylinder to your dive shop so it can be refilled. It’s well worth calling ahead, not just to ensure the shop is open, but also to find out if they can fill to your required pressure.
Some dive shops will fill your cylinder from an even larger cylinder, which looks like a torpedo. The massive shop cylinder, in turn, is filled by a compressor. One shop I used to go to would only refill its torpedo cylinder when the pressure was down to 240 bar, as the staff didn’t like firing up the compressor any more than they had to. I’d have to time my visits to coincide with ‘compressor day’ if I wanted a full 300 bar fill, which was very inconvenient as you can imagine.
Filling Your Rifle
Unless you have some sort of clamp or bracket attached to a wall, you should always lay the cylinder on its side for both storage and filling. If it topples over it could damage you, your rifle or the cylinder valve, with the gauge being particularly susceptible to impact damage.
Work out where you’re going to place your rifle, making note of the orientation and angle of the fill port. You can either lay the rifle down or stand it on a bipod when it’s being filled – and don’t forget to put the dust plug back in the gun when you’ve finished filling.
1. Mike’s preferred method is to fit the fill probe to the rifle before attaching it to the fill line using the quick coupler system – this means you’re not putting any strain on the rifle or the hose
2. The air fill line can now be connected by pulling back on the spring-loaded knurled ring on the quick coupler and snapping it into place over the quick coupler plug
3. With the rifle properly supported and the cylinder lying down, the gun is now ready to be filled, taking care to keep the air fill line hose as straight as possible – this helps airflow and minimises strain on the components
4. Close the bleed valve, but don’t overdo it – despite the very high pressures involved, modern valves seal incredibly efficiently, and turning them too tightly will just apply unnecessary force to the seals
5. Ensure you can clearly see the needle on the air gauge, then open the main valve slowly, taking care to close the valve when you’ve reached the desired fill pressure. You’ll also need to open the bleed valve before your can remove the air fill line, as this will still be under pressure, even with the main valve closed
Prepping Your Cylinder For A Refill Or Test
When the pressure in your cylinder has dropped below a useful level, you’ll have to take it back to the dive shop for a refill. The cylinder will also need to be taken back for a test, every five years in the case of a surface-use steel cylinder.
Remove your air fill line before you take your cylinder to the shop. The staff at the shop don’t need it on, and it can’t get lost or damaged if it’s left at home.
Your cylinder will have to undergo a periodic test to ensure it’s still safe to fill and use at the very high pressures demanded of it. This test involves not only the cylinder, but the valve as well. Unless a cylinder has been passed safe to use, where it’s said to be ‘in test’, the staff will quite rightly refuse to fill it.
The whole point of shooting airguns is to make use of high-pressure air, and having a charging cylinder makes the filling process quick and easy – so it’s worth getting familiar with the process as we’ve outlined it here.