Some precharged pneumatic basics explained
by B.B. Pelletier
Before we start, I have another plug about the Friday Facebook event from 10 to 11 a.m., Eastern. I'll be answering airgun questions on Facebook on this Pyramyd Air Facebook page. To see the discussion, you must have a free Facebook account. You do not have to be a recognized Friend of Pyramyd Air to ask a question.
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Today, I want to explore some of the basic facts about precharged pneumatic operations, because I sense the time is right. Let me begin with the term precharged pneumatic.
What is a precharged pneumatic airgun?
A precharged pneumatic (PCP) is an airgun that is filled with air and then shot many times before refilling. Compare that to a multi-pump pneumatic that is pumped many times by a built-in pump and then fired just once. To take another shot, the multi-pump has to be pumped up all over again.
How many shots each precharged pneumatic gets on a single fill of air depends on just one thing: How much of the stored air is used for each shot? Big bore airguns use incredible amounts of air and, therefore, get very few shots per fill. A Quackenbush .457 Long Action rifle gets two good shots per fill; on mine, the max fill pressure is 3,500 psi. After the second shot, the gun is down to 2,200 pounds per square inch (psi). My rifle gets about 560 foot-pounds of muzzle energy on the first shot and 490 foot-pounds on shot two.
A .50-caliber Career Dragon Slayer can get 5 good shots on a 3,000 psi fill. That rifle generates just under 200 foot-pounds on the first shot and drops off to about 120 foot-pounds by the final shot. And, once again, the pressure in the reservoir will be down to somewhere around 2,000 psi when you're finished. Exactly where it will be depends on how many shots have been fired.
That should answer another question that's often asked: Should you get a scuba tank or a hand pump to fill a big bore? The answer is "neither." To fill a big bore airgun, you really need a carbon fiber tank. I will explain all of this, but right now I need to back up, because this is report is supposed to be basic.
What is a hand pump?
A modern high-pressure hand pump is a mechanical pump that enables a shooter to fill a pressure vessel with air compressed to a very high level by muscle power, alone. Being mechanical, the pump requires effort; and, as the pressure increases, the pumping effort increases with it. From zero up to somewhere above 1,500 psi, the effort is relatively easy and most able adults will have no difficulty pumping. Above about 1,500 psi is where the effort starts to become noticeable. When I say that, I shudder because people come in all shapes and sizes and there is no such thing as a standard person. So, perhaps I should say that above 1,500 psi is where I begin to notice an increase in effort. I once watched an adult woman struggle to pump over 1,600 psi, so please take what I say in that light.
Also, hand pumps fill guns slowly. Think of this. At the tire store the hydraulic lift hoists your car rapidly and with great ease. Now, you try to do the same thing at home with a hydraulic hand pump bought at the hardware store. It goes a little slower, wouldn't you say? That is the difference between filling an airgun from a tank and filling it from a hand pump. If all you need to do is change one flat tire, a hydraulic hand pump is a great little tool. But if you're running a tire store, you want to have five or six bays, each with its own hydraulic lift. If you're going to be a serious PCP shooter, you will need serious air.
It can take 100 to 150 pump strokes of a hand pump to fill a big bore air rifle reservoir. It all depends on the size of the rifle's reservoir.
A big bore gets two to five shots from a fill and drops down to 2,200 psi in the reservoir, then it has to be filled up to between 3,000 and 3,500 psi to shoot again. That happens to be the hardest place for a hand pump to operate; and if it takes 100-150 pumps to refill the gun, what do YOU think about using a hand pump on a big bore? Maybe, only if it's your absolute last alternative? And, yes, I have done it a couple times.
Refilling a smallbore PCP
A smallbore air rifle comes in any of four calibers: .177, .20. .22 or .25. While there are still a great differences among these guns in the amount of air they use per shot, none of them uses anywhere near the amount used by a big bore. So, a smallbore gets many more shots per fill than a big bore. The most powerful guns of the bunch get the fewest number of shots because they use the most air. The AirForce Condor is one of the most powerful factory-made smallbores and has a special valve to extract the maximum number of powerful shots per fill. It also has one of the largest air reservoirs on the market. A Condor can get about 20 shots on a single fill when the power is set to its maximum.
Stepping down in power to an Air Arms S410, you may get up to about 35 or even 45 shots on full power. The actual number depends on the distance at which you're shooting and the velocity variation you can tolerate. Given that many shots, a hand pump may be a viable option for the shooter who is in shape, doesn't mind a little work and takes a while to shoot all those shots.
But in 2008, Benjamin brought out the Discovery rifle. It's a low-cost PCP that operates on just 2,000 psi. It's much easier to fill from a hand pump than most of the guns on the market. The Discovery gets about 25 shots from its fill. Not only is the work easier, but there are also a decent number of shots when you're done. The Discovery is a PCP that's designed to be filled by a hand pump. But if you use a scuba tank to fill one, you'll still be able to fill your Discovery all the way after other PCPs have drained the tank to the point that it needs to be refilled. That's another bonus.
A scuba tank
We talk about scuba tanks as though they are all the same, and they aren't. They come in different sizes and have different fill pressures, all of which affects the amount of air they contain. One very common scuba tank is an aluminum 80-cubic-foot tank. That means that the tank holds 80 cubic feet of air, not that the tank has an internal volume of 80 cubic feet. Since air compresses, what they are talking about is the number of cubic feet of air at sea-level pressure that is being filled into the tank. Since this particular scuba tank is rated to 3,000 psi, it can hold 80 cubic feet of sea-level air, when that air is compressed to 3,000 pounds per square inch (psi).
The pressure of air at sea level is about 14.56 psi. That number divides into 3,000 just about 206 times, so each cubic foot of air is being compressed about 206 times to get 80 of them into this scuba tank. By the way, that's where the pressure unit bar comes from. So, 206 bar equal 3,000 psi.
There are other types of scuba tanks. I have a couple little ones that hold only 6 cubic feet of air at 3,000 psi. They're used just to top off a gun during a match or when hunting. There are 120 cubic-foot, 3,500 psi steel tanks that aren't much larger than an 80 cubic-foot aluminum tank. Because they're steel, they hold higher pressure safely, so the same volume holds half again as much air (120 cubic-feet compared to 80 cubic-feet). I used to own a scuba tank that held air pressurized to only 2,200 psi. It was useless for filling most PCPs, with the exception of the Benjamin Discovery. So, don't think that all scuba tanks are the same.
Carbon fiber tanks
A carbon fiber tank is a breathing tank that is not for underwater. They're used mostly by rescue workers such as firefighters. Like scuba tanks, they also come in sizes, although their fill pressures tend to not vary as much. An 88 cubic-foot, 4,500 psi carbon fiber tank holds only 8 cubic-feet more air than an 80 cubic-foot scuba tank, but it does so with a very important difference. Since the carbon fiber tank is pressurized to a higher level than a scuba tank, it has more high-pressure air available. Consequently, there are a great many more full fills for any given gun in one of these carbon fiber tanks than in a scuba tank. You might be able to fill a PCP to 3,000 psi two times from a 3,000 psi scuba tank, and after that the next fill might end at 2,975 psi. After that you'll stop at 2,925 psi, then 2,850 psi and so on.
But, a carbon fiber tank that's pressurized to 4,500 psi will continue to fill a PCP to 3,000 psi many times. Perhaps, as many as 18-20 times, depending on the gun. So, we say the carbon fiber tank has about nine times more full fills in it than in a typical scuba tank. That's why the carbon fiber tank is so valuable.
Carbon fiber tanks have an aluminum bladder inside the carbon-fiber winding. Since the carbon fiber strengthens the bladder so much, the aluminum can be thinner and yet withstand even greater pressure. Therefore, an 88 cubic-foot carbon fiber tank weighs only about half as much as a scuba tank. That's a lot more full fills for your guns at half the weight. What's not to like? Well, there's the additional cost of the more expensive carbon fiber tank; but if you can get past that, there are very few reasons not to get one.
Why higher air pressure doesn't make your PCP shoot any faster
Think of a PCP gun as a car. Put in the correct fill, let's say 3,000 psi, and the gun shoots fine. As the pressure drops it still shoots fine because that is how it is engineered to operate. Fill a car with gas and it will go far and fast. But, try though you might, you cannot put in five more gallons of gas than the tank is designed to hold. Even if you could, the car would still go the same speed. It's not designed to go any faster.
Put a longer barrel on a PCP and it probably will shoot faster, just as taller tires will make a car go faster. But there are limits. Too long a barrel is ungainly, just as too-tall tires handle poorly.
How many shots can I get?
This question commonly comes from someone who is looking at their first purchase of a PCP with the same enthusiasm as an insurance underwriter looks at smokers' lives. Do you want the baritone to sing a moving song or are you just interested in how much of the alphabet he can burp?
What I mean by that is this. Shooting accurate shots is a goal. Hunting with clean kills is a goal. Shooting a PCP for as long as it will still poop out a pellet is a college prank. There's no useful purpose to that number, but a nickel-sized 75-yard group is appreciated by everybody. Find out what the gun you want will really do by asking those who really do it on a regular basis. Forget the online wizards with their tin-can technology and witches-brew lubricants that promise you Nirvana for $89, plus shipping.
Now, it's time for all of you readers who are prospective first-time buyers of precharged airguns to do your part. I really want to hear your questions about PCP guns, Don't worry about embarrassing yourselves because you don't know everything. Around here, we wait until you're an old hand and comfortable with us before we start embarrassing you.