HAM Airgun Technical – How PCP Airguns Work
In his first first article for HAM, our new Technical Editor Bob Sterne writes about how PCP airguns work and how they fit into the family of airguns. Welcome Bob and take it away…
Let’s Talk About Pneumatics
Broadly, there are two types of airguns; springers (including gas rams), and pneumatics. The former compresses the air during the shot, creating heat and pressure, with the heat further increasing the pressure and efficiency. Pneumatics use air or some other gas.
Let’s explain how PCP airguns work by starting with that gas…
Gas (eg. air, CO2 or Helium) is stored under pressure in an airgun. It releases some or all of that pressure to propel the pellet.
Since the gas is expanding, it cools, which actually reduces the efficiency. This is most dramatic with CO2 , which is stored as a liquid and “boils off’ to create more gas as you shoot. If you shoot rapidly, this cooling is dramatic, and really reduces the pressure and therefore the velocity of the shot.
You can see this reduction in FPS in the test targets of many of HAM’s comprehensive CO2-powered pistol tests, particularly where the guns also use the CO2 to operate the blowback action.
In fact, all pneumatic airguns cool as you shoot them. Powerful PCPs can appear to discharge a “fog” on a cold day as the water vapor in the (surrounding) air condenses out.
Pumpers create a bit of heat when you pump them, and this can cause them to lose some pressure and velocity if you let them cool before shooting. Pre-Charged Pneumatics (PCPs), generally are filled long before they are shot. So the temperature change is small, but enough that the barrel may feel cool to the touch after several shots.
Pumpers like the Benjamin 392/7 (above) most commonly, use a “dump valve”. This is filled with air as you pump. When you fire the gun, all of the air is discharged with every shot. This simple mechanism leads to very consistent velocity, at the expense of efficiency, since any air exiting the valve after the pellet leaves the muzzle cannot add to the velocity.
There is an exception, however. Some pumpers are “Retained Air Pumpers” (RAPs), also known as “Air Conserving Pumpers” (ACPs). They operate similarly to how PCP airguns work, in that only a portion of the compressed air is released with each shot. It can be argued that an ACP is really a PCP with an onboard pump!
CO2 guns typically retain most of their liquid in the reservoir (cartridge). Only the small amount which has converted to a gas and stored in the valve is discharged with each shot. Most pneumatic airguns use some variation of a valve like this:
The valve is held closed mostly by the pressure of the air (HPA), assisted by a spring. It is opened by being struck by a hammer or striker in the direction of arrow. This allows some of the air in the reservoir to escape through the port (P) and then into the barrel to drive the pellet.
Modern PCP airguns work by using very high pressure air (or sometimes Nitrogen or Helium), stored (precharged) in a reservoir. That reservoir can be a tube under the barrel, or a small bottle attached to the gun. Alternatively, PCPs can be “tethered” to a large tank, usually through a regulator to maintain constant pressure, to allow a huge number of shots. Our heading photograph shows an example of this.
Now Some Examples
Here is an example of a .257 cal PCP which I built from scratch, with a tubular aluminum reservoir.
When I built that gun, I also built a 6mm version running on a regulated 500 cc carbon fiber bottle. Both these PCPs shoot bullets, not pellets, and are intended for long range Varmint hunting.
The .257 is unregulated. The reservoir is filled with high pressure air (HPA) to 3800 PSI, and the gun is tuned to provide a short string of shots at very high power (nearly 200 Ft/Lbs).
Most of the time I would tether the gun to an 88 Cubic Foot, 4500 PSI SCBA tank (like Firemen use) through a regulator with a 3800 PSI output. This allows many shots. But if I need to, I can disconnect it and have a few useful shots without being tethered to the large tank.
The 6mm, on the other hand, is regulated at 2800 psi, and gets quite a few shots at about 130 Ft/Lbs before I have to refill the 500 cc carbon fiber bottle back to 4350 psi from my SCBA tank.
This brings us to how the two basic types of PCP airguns work – regulated vs. unregulated.
Using a regulator to reduce the pressure in the reservoir to a lower, but constant, pressure is somewhat easier to understand. Logic tells us that if the pressure is always the same for every shot, the velocity should be as well, and this is indeed the case. When we are talking about the consistency of velocity in a PCP, we use the term “ES”.
This stands for the “Extreme Spread” in velocity, and can be given in feet per second (FPS), or as a percentage of the highest velocity. I almost always use the latter, and you will constantly see me refer to a 4% ES, or a 2% ES, to describe the range of velocity spread.
Good regulated PCP airguns work to hold the velocity within less than 1%. But by the time you add pellet variation into the mix (pellets can often vary 1% or more in weight within one tin), most regulated PCPs run between 1-2% ES. If the ES is over 2%, you have more work to do!
This is what you can achieve when regulated PCP airguns work well:
Unregulated PCPs, to the novice, seem a contradiction! On the face of it, filling a gun to 3000 PSI, and then shooting it down to 2000 PSI, it would seem impossible to achieve a constant velocity.
Thanks to the commonly used “knock-open” valve (above), we can actually make PCP airguns work by tuning them to have a relatively narrow ES over a wide pressure range. We do this by tuning to create a “bell-curve” of velocity, where the velocity starts off below the peak, rises slowly, and then decreases again, like this:
A 4% ES is quite easily obtained, and by using a narrower range of pressure between fill and refill you can reduce that to 2%, or even just 1%. Just like a regulated PCP.
While the velocity isn’t completely constant, a 4% ES is similar to the velocity variation in a box of .22LR rimfire cartridges, and in fact even the best .22LR Target ammunition exceeds a 2% ES.
The big advantage of an unregulated PCP is that you don’t have to give up the high pressure the gun is filled to by reducing it through a regulator. So you can generate more power.
We measure the power of a PCP in Foot Pounds of energy, abbreviated as Ft/Lbs. In a PCP, pressure is power. All other things being equal, if PCP airguns work at 3000 PSI, they can produce about 50% more power than a 2000 PSI gun.
The other governing factor in the maximum possible power of a PCP is the barrel volume. The larger the barrel volume, and the higher the pressure, the greater the potential power a PCP can develop. You can get barrel volume from a larger caliber, a longer barrel, or both.
I will examine how those two factors in PCP airguns work to govern PCP power. That will be my next article…