A special thanks to guest blogger Jason “nervoustrigger” from the Gateway to Airguns forum for allowing us to share his article.
Introduction to PCP airguns
by Jason (nervoustrigger)
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, a man 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. Another fine resource is Gatewaytoairguns’ members map available at https://www.zeemaps.com/map?group=962067
(Editor’s Note: the AirgunsARP.com Facebook Group is another great resource)
Types of PCP airguns ‐ unregulated versus regulated
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.
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%.
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 and shot count.
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. (Tony@AirgunsARP – With high capacity carbon fiber bottles starting to appear on big bore PCPs, we are starting to see an increase int eh use of regulators for Big Bores)
Tony@AirgunsARP – Thanks to Jason “nervoustrigger@gta” for allowing us to share this with our fans. In upcoming guest posts, Jason will cover the basics of different pellet types, as well as how to properly fill and maintain your PCP Airgun.