[Sticky] Introduction to PCP airguns

Hey guys, this document is an introduction to the usage and maintenance of a pre-charged pneumatic (PCP) airgun.  It was originally written to accompany a rifle I built for my uncle, someone well acquainted with powderburners but not particularly familiar with the capabilities of modern airguns, so it also goes on to include some general airgunning topics like pellet selection and hunting and shot placement.  The intended audience is new PCP owners or those looking into perhaps purchasing a PCP.

What is a PCP?

A PCP airgun consists of a vessel containing high pressure air.  Firing the rifle causes a valve to release a finely metered volume of air to propel a projectile, generally a lead pellet.  It may be possible to get anywhere from several shots to hundreds of shots before it is necessary to refill the reservoir.  That depends mostly on the size of the reservoir, its maximum pressure, and the energy level to which the rifle has been designed.

Amongst the various types of airgun powerplants commonly in use, a PCP is regarded as having both the best accuracy and the highest energy potential of them all.  Those benefits generally come at a higher cost and the fact a source of high pressure air is required.

Like other types of pneumatic rifles (e.g. multi-stroke pneumatic or single-stroke pneumatic), there is relatively very little recoil compared to spring- or gas-piston airguns.   As a result, experienced shooters of rimfire and centerfire rifles will find a PCP easy to shoot accurately because they can generally be held firmly.  Also, whereas piston airguns tend to require very robust scopes to survive the repeated 2-way whiplash effect, a PCP airgun can be fitted with any scope.

The release of high pressure air can make PCP airguns quite loud unless equipped with a shroud or similar device which provides a controlled expansion of air as it exits the muzzle.  The very popular Benjamin Marauder is one such example.  Therefore prospective owners that have sensitive neighbors may want to do their research and perhaps test fire the gun (or one similar to it) before purchase.  Airgun enthusiasts are very open to sharing their hobby with others so if you do not have a brick and mortar store nearby, you may want to check online forums and post in the Welcome section. 

Types of PCP airguns - unregulated versus regulated

Unregulated
The most common type of PCP rifle is the unregulated type.

With each shot, the pressure in the reservoir falls.  If all parameters were to remain equal, that would mean each successive shot will be slower than the last.  However the dynamics of the system balance the lift and dwell of the valve to produce a fairly consistent velocity over a wide range of pressure.  This behavior is referred to as self-regulation.  The result is a bell-shaped velocity curve similar to the following graph:

Notice the velocity covers a range of roughly 750fps to 800fps, or a 50fps spread.  That represents a 6% extreme spread (50 / 800 = 0.0625) which would provide a suitably stable trajectory out to 30 yards or so.  For longer distances, a tighter velocity spread would be desirable.   A common recommendation is 4% for 50 yards or 2% for 100 yards.

So whereas this hypothetical rifle would give about 40 “good” shots out to 30 yards, it would be good for about 30 shots at 50 yards or 17 shots at 100 yards.  Granted, 100 yards is a long poke for a pellet rifle, for reasons having less to do with velocity and more to do with the ballistic coefficient of a pellet.  But that is a subject for a different day.

Professional airgun tuners and DIY owners alike may spend a lot of effort adjusting a PCP to deliver a particular balance of energy and shot count.  To me, that is one of the most compelling advantages of a PCP—that it can be tuned for maximum power for shooting and hunting at longer ranges, or tuned for high shot count for recreational shooting in the back yard.  Tuning is beyond the scope of this document but be advised there are many online resources and guides available, many times for the particular make and model you have.

Regulated
To improve consistency, a regulator can be used.   It maintains a stable pressure at the valve, making it possible to achieve a very uniform velocity for the entire shot string.  It is not uncommon to achieve an extreme spread of 1%.  For example, it may look like this:

So why don’t all PCP rifles use a regulator?  As you probably guessed, one reason is cost.  While a regulator is not terribly expensive, many airguns are in a very price sensitive category.  Therefore regulators are seldom found in entry-level PCP rifles, although some owners will install an aftermarket regulator to improve the consistency.

 The other reason is energy.   A regulator can only produce a pressure lower than that of the high pressure reservoir.   For larger calibers, that tends to limit maximum energy to an unacceptable level so regulators are often used in .177 and .22 cal rifles and sometimes .25 cal, but seldom with calibers larger than that. 

Filling up a PCP rifle

General info – Filling a PCP rifle is done one of two ways, either a tank or a pump:

The luxury answer is to purchase or rent a high pressure tank such as a SCBA tank or industrial nitrogen tank.  Do NOT use pure oxygen to fill a PCP because of the risk of explosion.  Keep in mind if your rifle uses a 3000psi fill pressure, you need a tank with a higher fill pressure or you will not be able to fully refill your rifle.

However since this is an introductory guide, we are instead going to focus on the other method:  a high pressure pump.  This type of pump is like a bicycle pump on steroids.

Connecting
The most common type of connection is a Foster quick disconnect.  Others use a fill probe.  They may differ slightly from brand to brand but in general look similar to one pictured:

The Foster connector is similar to the quick disconnects used on a typical shop compressor, having a spring-loaded collar that locks the two fittings together.  The key functional difference is that, for safety reasons, the connection cannot be released under pressure.  You must first bleed the pressure from the line.

How to pump
The best technique for pumping, particularly at pressures above 2000psi, is to use your body weight.  Basically what you do is almost lock your elbows, then let your knees give out such that most of your body weight is dropped suddenly to compress the pump.

Pump slowly, allowing 1 second at both the top and bottom of the stroke so as to allow time for the air to move from one pump stage to the next.  Pump in sessions of approximately 50 strokes.  Allow the pump to cool for 15 – 20 minutes between sessions or the seals may prematurely fail.  Between sessions, bleed the pressure from the hose, usually via a brass thumbscrew on the base of the pump.  Open it briskly to allow the air to escape quickly.  Doing so helps seat the check valve in the airgun.

Fill to what pressure?
A typical fill pressure is 3000psi (approx. 200 bar), however some will range anywhere from 1500psi to over 4000psi.  Do not exceed the manufacturer’s maximum safe working pressure (MSWP).

Depending on how the gun is tuned, it may be best to stop filling somewhere shy of the MSWP.  That is, the pressure which corresponds to beginning of the useful portion of the bell curve.

Similarly, if your airgun has a regulator, you may choose to only partially fill it.  As long as you fill to something above the regulator setpoint, the gun will operate at a consistent velocity.  Let’s say you have a regulator set at 1000psi and you can get 60 shots on a complete fill of 3000psi.  If instead you filled to 2000psi, you would get 30 shots.  Since pumping gets substantially more difficult as you approach 3000psi, that can be a real advantage.

Pump maintenance
Occasionally withdraw the pump handle and check the exposed tube for a light coating of lubricant.  If it is dry or if the original lubricant becomes dirty and contaminated, clean the tube with a paper towel and reapply a light film of 30W silicone oil.  An O-ring rides the tube so anything more will simply get wiped away on the first stroke.

Check the manufacturer’s recommendations for other types of preventive maintenance and lubrication.  In general they don’t recommend internal lubrication but some users like to lubricate the innermost piston rod and its O-ring from time to time.

Filtering and drying the air
Just like in a shop compressor, moisture from the air will condense inside the pump.  When you bleed the pressure, you will likely notice a blast of water vapor.  Unless you do something to prevent it, over time it will corrode the pump’s internals and may cause rust in your gun as well.

Some pumps have an integral filter & drier.   Typically these devices are about the size of a D-cell battery.  It is better than nothing but if you live in a humid climate, it’s not a bad idea to install a larger desiccant drier on the pump’s intake to remove more of the moisture.  Something like a quart jar filled with desiccant beads and plumbed with flexible hose to the pump’s inlet will do a good job at protecting your pump and guns.

Pictured below is an adaptation of GTA member tenring’s original concept.  It uses a 1L water bottle but you could use plastic quart jar like mayonnaise comes in.

Note some pumps have the inlet on the base and others—like the one pictured--have it in the hollow handles.

After a period of use, the desiccant pellets will become saturated and less effective at trapping moisture before it goes into the pump.  If you want, you can use a color-changing desiccant to recognize when it needs to be dried.  To dry the desiccant, spread it out on a cookie sheet and bake it at 225 - 250°F for 2 to 4 hours.  A toaster oven is perfect.

Lastly, be sure to keep the container capped when not in use, otherwise it will absorb moisture from the environment. 

Gun Care

Cleaning the barrel
Contrasted with powder burners, air rifles do not require frequent cleaning.  There are no combustion byproducts and the pressures and speeds are much more benign.   Generally every 500 shots or so is sufficient but factors like a rough bore, a tight choke, and/or speeds 900fps and higher may require more frequent cleaning.  Therefore as a general rule, it is better to clean only when you notice a degradation in accuracy.

Some pellets are lubricated with graphite powder to prevent oxidation.  This powder will leave dark marks on your fingers and will coat the inside of the barrel.  However it is largely benign in terms of fouling the barrel.  However over time, lead will build up on the rifling and affect accuracy. 

It is not necessary to use a brass brush to clean an air rifle barrel.  All you need is a pull-through and cotton patches.  Little squares cut from an old T-shirt work well, better than store-bought patches in my experience.  A DIY pull-through made from string trimmer line will get the job done.  Simply melt a small ball on one end and sharpen the opposite end.  Load a patch onto it and apply a drop of your favorite cleaner.  Push the string through the barrel from breech to muzzle, and then pull it on through.  If your rifle has a shroud, the string may hang up as you attempt to push it through.  A simple drinking straw can be used to guide the string. 

Continue pulling patches until they emerge clean and then finish up by pulling a couple of dry patches. 

Opinions vary quite a bit on what cleaner to use but a popular one is Goo-gone.  It’s usually easy to find at dollar stores.  If you want to deep clean the bore, WD-40 works well as far as commonly available cleaners are concerned but I prefer Kano Kroil.  Another very effective cleaner is Shooter’s Choice.  Some will argue this type of cleaning is unnecessary but if you notice a loss of accuracy and can’t seem to determine the cause, deep cleaning may be worth a try.  All I use is Kroil.

The general recommendation is to never use steel cleaning implements on an air rifle barrel.  The steel used in air rifle barrels is usually a softer alloy compared to barrels made for powder burners so be attentive if you use a cleaning kit meant for rimfire and centerfire.

Degassing - emptying the rifle for maintenance
It has been said there are two kinds of PCP airguns: those that leak and those that haven’t leaked yet.  It is inevitable that a PCP will eventually need servicing.  Replacing O-rings is well within the capabilities of most people but high pressure air is potentially deadly so you want to be sure you do not attempt to disassemble a pressurized gun.

Remember previously where we talked about releasing the bleed screw briskly to help seat the check valve?  You can use this knowledge to your advantage if you need to degas.  You can open the bleeder screw very slowly until it just begins to hiss.  That will allow the air to escape slowly by allowing the check valve to remain open.  You’ll need to gradually back out the bleeder screw a few times to allow air to continue to escape as the pressure falls.  Once the pressure is down to zero—or appears to be down to zero—dry fire the gun to be sure.  For the vast majority of PCPs, dry firing is okay but it’s a good idea to double check before doing so.  Again, the forums are an excellent resource if you are uncertain.

If the gun is a regulated type, there will definitely be air still in it because the regulated plenum is isolated off from the main reservoir.

Lastly, be aware there are some fill ports that use a spring-loaded check valve.  These types of guns must be degassed by a different method.  Some guns will include a degassing tool.  Others will require a bit of ingenuity.  Generally speaking, dry firing is a way to do it if you don’t know a better way.  It just tends to be slow and perhaps noisy.

Pellet Selection

Just as with any rifle, the ammunition is a critical component to obtaining good accuracy.  With air rifles, this concept cannot be stressed enough.

The right pellets make all the difference in the world.  One type may produce bughole groups at 25 yards and another may scatter over an inch.  Once you find one that works well, it is a good idea to go ahead and buy a few tins.  Manufacturing tolerances vary and dies wear out so those you buy next time may give you a much different result than those you buy today. 

Pellet types

Wadcutters – For shooting at close range and low velocities, wadcutters (flat-nosed) pellets usually work very well.  However they tend to destabilize beyond 25 yards or so, or when fired at high velocities.  Occasionally that rule gets broken but it’s a good generalization.   Whereas other types of pellets tend to tear paper targets, wadcutters create nice crisp holes which make the targets easy to score.    They are also devastating to birds and small game because the flat helps to dump all of the pellet’s energy into the critter (rather than passing through).  Think high speed frying pan.  But they are not recommended on medium game like raccoons in which a brain shot is needed for a quick, humane kill because the flat profile may have difficulty penetrating bone.

Domed – Simply put, domed pellets are the best choice for most types of shooting.  They have the best ballistic coefficient and therefore tend to give the best long-range accuracy and energy retention.   In terms of hunting performance, at muzzle energies of 20fpe or higher (in .22 cal), they are capable of penetrating the notoriously hard skull and cartilage of an adult opossum at least out to 50 yards.

Pointed – Pointed pellets are claimed to offer better penetration.  That may be helpful if you are using a low-powered rifle, however the legitimacy of that claim is called into question by actual tests in ballistic gel.  In most guns, they have no practical benefit over a domed pellet and will tend to have poorer accuracy at almost any range.

Hollow points – In general, hollow point pellets do not expand a great deal on impact with flesh due to a variety of reasons.  Airgun velocities don’t promote expansion, some of the pellet alloys are too hard, and some simply have an inadequate geometry to actually expand.  Some exceptions are the H&N Crow Magnum and Skenco Ultra Shock.  If your gun groups a hollow point well, use it.  Not so much because of its expansion but for the accuracy.

Hybrids – There are a number of hybrid pellets on the market that have a 2-piece construction.  Most have been shown to be gimmick but one noteable example is the Predator Polymag.  It has a pointed plastic tip and tends to be accurate in many rifles.  On impact, the plastic tip sheds and the remaining projectile has a hollow point geometry which actually mushrooms.  The result is a devastating hunting pellet.  But as you might expect, they are expensive…about twice as much as a quality domed pellet.

 
Hunting with an air rifle

For the most part, responsible air gun hunters will opt for brain shots or heart and lung shots to ensure quick, humane kills.  Small caliber pellets do not have nearly the energy of a rimfire rifle (e.g. .22LR), and certainly not the supersonic speeds and accompanying hydrostatic shock of a centerfire. 

Therefore shot placement is extremely important by comparison.  Most hunters can attest to the toughness of a tree squirrel or a ground squirrel, having seen how they can sometimes take a direct hit to the vital organs from a .22LR and yet run off and disappear.  That might be in the vicinity of 100fpe, yet hit the same animal in the brain with a .177 pellet at 12fpe and it is instant lights out.  It is for that reason I prefer brain shots on furry creatures.

Power without accuracy means nothing which is why it is important to find your airgun’s magic pellet...

The Magic Pellet
There is no way to reliably predict which pellets will shoot well in a particular rifle.  You just have to try different types until you find one that groups well.  I usually sample 8 or 10 different pellets to determine which ones group the best.  If I’m fortunate, I may find 2 or 3 that work well in a particular rifle but more often than not, there is one that is distinctly better than all the rest.  That is what air gunners refer to as the rifle’s magic pellet.

Here are some example groups shot by a .22 cal QB78 rifle that has been modified to use a regulated bottle.  At 25 yards, Crosman Premier Hollow Points group well enough to be used for hunting. However, at longer range the groups tend to open up.  At 40+ yards, the JSB Jumbo Heavy 18.1gr perform better. 

It is very rare that two different pellets will have the same point of impact (POI).  Therefore if you switch between pellets you will want to re-zero the scope.

Purchasing pellets
For most of us, availability of pellets locally is rather limited.  Online sources like Pyramydair.com or Airgundepot.com or Amazon.com have a much better selection of quality pellets.  But be advised that pellets can be damaged in shipping rather easily if they are not packed adequately.  That has been my experience with Amazon.  Sources that cater to airgunners like Pyramyd and Airgundepot do a much better job of getting pellets to their destination safely.

Closing Words
My best advice is to set up targets at varying distances and practice.  Shoot groups and learn the holdover (or holdunder) required at different ranges.

Keep in mind that the amount of holdover/under compensation varies with the magnification setting.  Well, unless you are using a first focal plane scope, in which case you probably know already.  A very useful piece of free software for evaluating trajectory under different conditions, including angle of incline, is Hawke’s Chairgun.  Following is a screenshot of trajectory out to 100 yards for the rifle that shot the groups above.   Often times, the actual holdover values match very closely to that predicted by the software but it’s always a good idea to check at the range.