How to Fill Pre-Charged Air Rifles

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I wrote about modern airguns before and whether PCP air rifles are worth the money. One topic was whether these rifles have adequate power to humanely harvest small game. This criteria rules out most air pistols, especially springers. For those who missed it, here’s a rehash of each design.

Regarding springers, many of today’s adult-type .177 or .22-caliber “pellet guns” are barrel-cocking designs. Upon discharge, a piston driven by a compressed spring generates a blast of pressurized air to expel a pellet. Inherent to the design, achieving sufficient power for humane small game hunting usually requires a large rifle.

Its discharge also creates a unique reverse-recoil impulse which can destroy a non airgun-rated optic. These dynamics can result in a “hold-sensitive” gun that requires consistent techniques for optimum accuracy. Greater power also requires substantial cocking effort – and the spring shouldn’t remain compressed, which complicates hunting.

The latest gas-piston barrel-cockers solve the latter concern, but power still translates to size –  and some advertised velocities are “optimistic.” On a positive note, a springer is a self-contained system.

Most of the drawbacks associated with springers disappear through pre-charged airguns. Their projectiles are expelled through release of high-pressure air from an onboard reservoir. Some big-bore .45 or .50-caliber types only provide a few shots before refilling becomes necessary.

But, a useful .22/800 fps small game hunting benchmark is easily attainable from a 6-7 pound gun capable of providing 25 shots or more. Today, many are even repeaters that feed from rotary magazines. Pricing of some PCP guns now rivals mid-grade springers. But a PCP gun does require extra gear, dependency on a fill-source, and the added expense of necessary equipment.

PCP advances aside, for those seeking a non-complicated, affordable, and self-contained airgun capable of pest control or small game hunting, a springer is still a good bet. Humane harvesting of gray squirrel-sized quarry requires a rifle with some heft and cocking effort, but many springers can achieve 800 fps (or more) using standard-weight .22-caliber pellets. With careful shooting (actually true of all airguns) that’s enough punch to do the job inside 40 yards or so.

However, many serious shooters seek maximum performance and enjoy the associated technical aspects. To scratch those itches the PCP field offers an intriguing new frontier. And here’s another twist; one I just encountered. While recovering from a recent surgery I was in need of some therapeutic backyard plinking.

Normally, I’d just grab a hassle-free springer. Trouble was, a temporary 10-pound weight restriction rendered it useless due to the necessary cocking effort. Same story for a trusty pump-up pneumatic. No worries though. Two fully charged PCP rifles were on hand, along with an easily accessible scuba tank.

After a rejuvenate 45-shot session spent ringing small steel targets from the back deck out to 100 yards, the gun was easily recharged through a twist of the tank’s knob.

Pre-charged Air Rifles – The Basics

First up, pre-charged pneumatic guns operate at some serious pressures! Even the lower-pressure types hold 2000 PSI. More are charged to around 2900 PSI (often expressed as 200 BAR), but some now run at 3650/250 BAR – or even higher! Air reservoir capacities also vary, but their volume is typically expressed in cubic centimeters, which could range from 100 to 500 CCs.

Larger reservoirs and higher pressures translate to more shot per fill. Repressurizing such a gun is accomplished via a special PCP hand-pump, a scuba tank, or an airgun compressor (see below).

Usually, a short length of high pressure air (HPA) hose provides the necessary connection between the gun and its air source. The fittings can vary, depending upon the air source and the gun. This can create some confusion for new shoppers (more to follow).

As for the guns, many can be pressurized by connecting the hose to a common Foster air-tool nozzle. Using this system, a female QD fitting on the end of the hose simply snaps on to the gun’s male nozzle (via a spring-loaded collar). Others are filled through small port that accepts a male probe.

My Air Arms guns employ a proprietary brass collar with a corresponding fill-port. For this reason most pre-charged guns ship with the necessary “fill adapter”. Fortunately, nearly all utilize the common 1/8 BSPP thread pattern.

How to Fill PCP Air Rifles

A PCP gun’s operating pressure and air capacity can have a bearing on the air source, something worth considering prior to finalizing a PCP purchase. Here’s why…

Hand Pump

As noted in the previous post, the special PCP hand-pumps closely resemble a heavy-duty bicycle pump. The difference is their performance! They do provide total air independence, but lots of exercise will be a byproduct, making smaller-CC guns are better choices.

Accumulating water can be another concern although some incorporate moisture traps. Recently, complete PCP gun and pump packages have appeared, often priced to attract newer shooters. For more casual use in smaller guns they’ll suffice, but higher-performance guns will probably need more. Especially when dealing with the latest 3500+ PSI guns, the final stages of pressurization require lots of effort.

The Hill Hand Pumps are longstanding favorites and some are now rated to deliver 4500 PSI/310 BAR! Given this level of performance, a Hill’s $270 price isn’t that far above average costs of around $200. Most also come with a properly matched HPA hose set up for a female QD fitting and the batter ones can be rebuilt using replacement parts.

Charging: Typically, a short length of HPA hose is connected to the pump’s outlet at the bottom of the pump. The other end is attached to the gun. A pressure gauge on the pump then monitors the charging process, which is accomplished through deliberate strokes.

The idea is to use the entire upper body while flexing at the knees – probably with a break or two thrown in. Recharging an empty gun could be brutal but, more often, partial depletion of shooting pressure results in less exertion. Either way, upon reaching the gun’s desired pressure level, a crucial step remains! Before disconnecting the hose, a small bleeder-valve must be cracked to vent potentially dangerous residual pressure.

The bleeder is usually a small brass knob located near the pump’s outlet and pressure gauge. After the hose is vented it can be safely disconnected from the gun. If the hose remains attached to the pump, it can be covered with a plastic bag to ward off dust.

The pump’s advantage is air independence in a reasonably-sized package (some folks also use one to augment a scuba tank when its pressure subsides). Its downside: Not everyone is up to the physical requirements. Even a smaller-sized PCP gun will probably require two or more pump-strokes per shot.

Scuba Tank

My larger-volume .22-caliber Air Arms 400 FAC offers 45 useful shots at 920 fps when fully charged to 2900 PSI/200 BAR. That translates to lots of hand-pumping. Given the price of the gun and pump-related moisture concerns, for me, a tank became the logical alternative.

Scuba tanks come in various sizes (50-150 CF), and are built from three main materials; aluminum, steel, or carbon-fiber. The more common flat-bottom, 63 or 80 CF aluminum versions are practical corrosion-free choices, the latter weighing around 35 pounds when filled to 3000 PSI (an equivalent steel version is actually lighter due to its thinner walls).

I bought a standard aluminum 80 cubic-foot tank “used” at a local dive shop and eventually snagged two more, the last one for $150 (I also brought the airgun to allay liability concerns). Most metal tanks including mine employ a threaded-in “K-Valve”. Beyond a main on/off control knob, this part is designed to quickly connect with dive regulators. But, by attaching a different manifold (known as a “yoke”), an HPA hose can be connected to this part.

Carbon-fiber tanks are surprisingly light but also quite expensive. These (and fire-fighting SCBA versions) typically employ a different-type “DIN-valve”; a threaded heavy-duty stem. Nevertheless, most well-stocked airgun dealers inventory the necessary PCP gun adapters.

Because they have a finite DOT-regulated 15-year life, a new CF tank is the best bet. Recently, reconditioned CFs became DOT-approved, however they require 5-year inspections via a specialized Colorado-based source (which for most of us will involve shipping).

All-metal tanks need an annual internal visual inspection, and a more in-depth “hydro-test” every five years. A further concern for aluminum tanks involves its manufacture date. Those built in the U.S. after 1988 use stronger alloys to prevent neck cracking. For this reason, mine all came from a trusted dive shop with current tests.

Speaking of dive shops, assuming one is in your area, it may be well worth a visit prior to investing in a gun. Some might not be equipped to handle every fill option. Because my shop’s compressor can’t reach the highest gun pressures (of 3500 PSI or more) any future gun shopping will be limited to 200 BAR types. But, that’s adequate for me and works with my existing tanks, some of which are now quite old.

If one finally flunks its inspection, well, good enough. Meanwhile, refills cost just a few bucks and, when pressurized to around 3200 PSI with pure, moisture-free breathing air, numerous gun refills are available. That’s because I often don’t shoot through the gun’s entire air supply. Assuming I consistently did so, Airguns Of Arizona’s handy tank calculator indicates I should be good for around 13 refills from my 80 CF tank.

Since each top-off provides 45 useful shots, that’s more than a tin of 500 pellets. At that point, within moments, I can transfer the yoke/hose assembly to a fully-charged spare tank. The empty can head to the dive shop when time permits. One kicker though: Right now I’ll need help carrying it!

Consult the tank fill calculator before filling.

Charging: Using a common K-valve aluminum tank as an example, attachment of the necessary yoke is easily accomplished using its thumb-screw. The yoke’s housing also makes the perfect spot for a pressure gauge and bleeder valve. My hose is threaded into the yoke, but I could’ve used a QD system identical to the fitting on its opposite end.

The male Foster-type stem of my proprietary Air Arms collar will snap into the hose’s QD female fitting. The collar then slips over a nozzle on the gun where a 90 degree twist will securely lock it in place under a T-bar. Next, the knob on the scuba tank is slowly cracked to decant air. The yoke’s pressure gauge is carefully monitored and, since even slow filling generates heat, a short cooling pause is followed by a final top-off.

Actual fill-time is still nearly instantaneous, so the tank is opened with caution. Upon reaching (never exceeding) the operating pressure, the tank’s air supply is turned off. The bleeder valve is then cracked (emitting a tell-tale hiss), and the hose is disconnected from the gun. I Leave the yoke/hose assembly connected to the tank and cover it with a plastic bag.

The charging accessories will add another $100 but, in my case, thanks to used tank pricing, the system’s cost was comparable to a decent pump.

Compressors

The standard shop-types won’t work, but an airgun model will do the job – for those willing to incur the expense. So far I’ve resisted one, although some aficionados own a compressor because it’s possible to a fill scuba tank as well as a PCP gun. A few newer types are also fairly compact and easier to handle than a full-size dive tank.

Prices have also decreased and some now start at around $650 but, of course, others cost much more. Some will run on AC or DC power so, ss long as you have a compatible power source, an infinite supply of filtered HP air will be available. Fill-time for an average airgun is several minutes but, of course, a tank can take longer.

Charging: The process is similar to scuba. Compressing large volumes of air can generate considerable heat which could damage the unit, so overload protection is usually incorporated.

Necessary Connections

Do you need male adapters or female types, and what about fittings? The choices are a bit daunting. I wound up buying the properly matched yoke and hose from the local dive shop that sold me the scuba tank. We assembled the parts in the well-stocked store and made sure everything worked before I left.

This happened 13 years ago when PCP technologies were still obscure. Given the various tank designs, rated pressures and valve systems (K or DIN), it’s still not a bad approach for those contemplating the scuba route. Or, if you do your homework first, another option is a well-stocked dealer who specializes in airguns.

Tech Support

Today, several on-line airgun dealers make great starting points. Among them is Pyramid Air. Scrolling through their website, you’ll see pumps, various tanks, and compressors. You’ll also see all kinds of HPA fittings and hoses, including some preassembled on pumps or scuba yokes. For what it’s worth, they also have an extensive airgun inventory so a phone call should point you in the right direction.

Some airgunners run multiple guns off one charging system. By using the correct QD hose fittings they quickly snap on different fill adapters. If assembling your own fill system, use Teflon tape on the threads to prevent leaks. Also, a very short hose can be inconvenient.

Other Factors

Remember, the highest-pressure guns require air sources up to the task. A gun can work if charged below its maximum pressure, but its performance will be hamstrung. As mentioned above, a few hand-pumps and some compressors can get there. Today’s lightweight carbon-fiber tanks can also take some serious pressure – like 4500 PSI –  but you’ll still need a way to fully charge one.

Not all dive shops have that capability, but some fire stations may. A few airgunners even use high-pressure SCBA tanks after adding an adapter to the valve. However, beyond the gun, fittings and hoses must also be properly rated!

Cautions and Concerns

High-pressure air is serious stuff. Forgetting to bleed a hose can be a memorable event. A dropped tank can turn one into a rocket; a hot trunk into a bomb! The gun wrong lubes can produce similar effects due to pressure-related heat ignition. Alternate gasses are also dangerous (some serious airgunners use nitrogen but I’m staying with air). Over-pressurizing a gun is bad and can even be counterproductive.

For me and many others, velocity becomes much more consistent at a bit below maximum safe pressure. Today’s increasingly popular regulated guns offer a consistent velocity spread throughout their useful charge, but even simpler non-regulated versions like mine can deliver sustained accuracy once optimum pressure parameters have been established.

A gun’s built-in pressure gauge (manometer) is the normal refill indicator. However, people in the know advise regulating the process using a fill source’s separate gauge. There can be disparities like the fill lag I’ve seen in one of Air Arms rifles.

Dust and debris are enemies that can cause slow leaks so it’s best to leave pressure in the gun. I store mine fully charged (or near that level) BUT the gun remains un-cocked! Inherent to the design, determining a PCP’s load status is difficult. The safest bet is to point it a safe backstop (absent a magazine for repeaters), cock it, and shoot. Unlike a springer, an occasional dry-fire (still dangerous at close range due to the HPA blast) won’t damage the gun. Also, be sure to observe muzzle discipline while performing refills!

As for bore cleaning, most folks including me use a flexible pull-through system. I just use a patch lightly moistened with Break-free CLP after every tin of 500 pellets or so. The gun is inverted to prevent introduction of foreign materials into the barrel’s air-transfer port. The same lube will also handle exterior metal surfaces. The O-rings that mate with my fill adapter receive a sparing film of diver’s silicone.

Lastly, at some point, a PCP gun may develop a leak. A weak point is the internal O-rings which, despite scrupulous maintenance, could eventually dry out. I’ve had that happen, but full function was restored thanks to the properly equipped dealer who sold me the gun. Much of the expense related to shipping, which can be the biggest hassle.

Conclusion

At this point, a springer could sound a whole lot more attractive! That said though, the quiet and recoil-free performance of a PCP gun, coupled with typically uncanny accuracy, can make for some fascinating shooting. Quarter-size (or smaller) 50-yard groups are the norm. I’ve experienced blips of inconsistent accuracy from powder-burning guns, but my air-powered pre-charged wonders always seem to be spot-on. Why? No barrel-heating or fouling… perhaps.

The absence of firearms-related stigmas and hassles is also refreshing. The Fed’s don’t regulate airgun purchases so we can even have ‘em shipped to our doors.