How a precharged pneumatic airgun works
by B.B. Pelletier
This report is specifically for Mannish, a reader from Mumbai. But I imagine we have many readers who might like to know the same things. How does a precharged pneumatic (PCP) airgun work?
First, let's describe the differences between a PCP and a multi-pump pneumatic. A multi-pump is a gun that one pumps full of compressed air before taking a shot. The compressed air is mainly stored inside the gun's firing valve. When you shoot, the valve exhausts all the air inside--if it is able.
Sometimes, we over-pump a gun, and the valve cannot open far enough or stay open long enough to exhaust all the air. That's called valve lock, but you might think of that gun as a precharged pneumatic, because we charged the rifle and then shot it more than once before charging again. After that first shot, the gun was still charged with pressurized air for another shot, so, for that shot, the gun was pre-charged. See where the name came from?
All you have to do is add a larger reservoir to this gun and you can store more compressed air. Instead of shooting just one extra shot, we might be able to get about 20 shots. And most of them will be at approximately the same velocity.
So, do we call that gun a precharged pneumatic? No, we don't. By convention, we call all guns that have air pumps attached "multi-pumps," unless they're single-stroke pneumatics, which I don't want to get into here.
Precharged pneumatics or PCPs are, by accepted convention, those guns into which pressurized air has been added from a separate device like a scuba tank or hand pump. That air is stored in a reservoir in the gun until the firing valve allows some of it to exhaust during a shot. Their valves work just like those found in multi-pump pneumatics. And, since there is too much air in the PCP reservoir for the valve to exhaust all at once, the gun gets many shots per charge.
Too much air to exhaust. How does that work?
Well, there are many factors that determine the efficiency of a firing valve, and I only want to discuss two of them. A firing valve's efficiency is controlled by the pressure in the reservoir behind the valve and by the amount of time the valve remains open. Now, I could go on for pages discussing things that control the amount of time that a firing valve remains open, because there are a lot of them. But for the sake of brevity, I will limit this discussion to just two things--the size of the valve and the strength of the hammer blow that opens it.
If one person holds a door closed against you trying to open it, you may be able to get it open. Of course, the stronger the other person is, the harder it will be. So let's make him a five-year-old, so that most of us will be able to open the door easily. Now, if we add a second child, the door will be a little harder to open, but no sweat, right? But what if there were 15 kids holding the door shut against you? Maybe then you'd find it difficult to open?
But 15 kids couldn't push against a door unless it was very big. So let's say that it's now 10 feet wide. Maybe with a door that big 15 kids could all push against it at the same time. And if they could, you would have a hard time opening it.
In my analogy the kids are pressurized air. And the door is a valve. If the door is only three feet wide maybe only four or five kids could push against it at one time and you would find it easier to open than with all 15 kids pushing.
If kids run through the opening while the door is open, eventually there won't be enough kids behind the door to matter to you. You'll be able to push the door open and hold it there and they will all run out. When that happens, the size of the door ceases to matter.
As the number of kids behind the door decreases (because some are escaping every time it opens), you'll eventually get to the point that no matter how large the door, there isn't enough resistance to keep it closed against your push. That's the point at which the valve drops off the power curve, more-or-less.
Let's remove the kids and put people as strong as you behind that 10-foot door. It's not going to open, is it? Nope, that door is valve-locked. So, putting more pressure inside the reservoir isn't going to boost your gun's power if the valve isn't sized correctly for it.
When the Benjamin Discovery was designed, it was determined that lower air pressure (smaller kids) could do the same work, as long as enough of them made it through the firing valve. With a long barrel there was plenty of time for the lower-pressure air to push on the pellet. Sure, five-year-olds aren't very strong, but get enough of them pushing and they can do the same work as a couple of strongmen.
A bigger hammer!
The other thing we can do to change the valve's efficiency is to swap you for a real strong man to open the door. Then maybe he can open a 10-foot door with 15 kids pushing against the other side. And, we can make the room on the kids' side large enough to hold hundreds of them, so now we have a more powerful "gun." But we lose a lot more kids every time we open the door, so we don't get as many "shots." But our big "hammer" lets us open the door farther than if we used a wimpier hammer.
All we have to do is design a really big room for the kids to be in. Oh, but that makes the entire building bigger, doesn't it? Well, what if we make the door smaller and fill a smaller room with stronger people? That would work, wouldn't it? Maybe the strongman (bigger hammer) could open a three-foot door with a few grown men pushing against it. The few grown men that escaped through the door while it was open would be able to do a lot of work on the other side, wouldn't they? Maybe as much as a lot of five-year-olds. We could have a smaller room for the grown men, yet we could get a lot of powerful "shots" from them this way.
Two things that can affect our valve are its size and the time it remains open. If we change the pressure inside the reservoir, that will also change the power of the gun, but very quickly it might get to the point where the valve doesn't work very well. That's because it was designed with a different hammer strength and a different valve/door size.
A PCP works by a balancing act of the valve size, hammer strength and the pressure inside the reservoir. As long as the pressure is within the design parameters of the valve, it works well. Once it goes outside those boundaries, the valve doesn't work as well.