Getting started with a precharged air rifle: Part 3
by Tom Gaylord
Writing as B.B. Pelletier
This report covers:
- Fill options
- Filling a big bore
- Tank size
- Filling a smallbore PCP
- The point
- Air compressors
- Booster compressors
- Stand-alone compressors
- The future
I said in the last report that I would write this report on filling options for PCPs. I’m writing this for the new guys who aren’t sure which way to turn. Any way you go represents an investment, so this is something that needs to be given a lot of consideration. Hopefully this report will start a discussion of that.
There are two basic ways to fill a precharged airgun. Either the air is introduced from a container where it is stored until called for or else it is put in and compressed as the fill is made. The first option is based on an air tank of some kind. The second is either a hand pump or a small air compressor that connects to the airgun. I will talk about both of these but first I want to discuss practicality.
There are many ways to get compressed air into an airgun. I just said there are two basic ways, but these break down into dozens of different possibilities. Some are practical and others aren’t. Let’s talk about that for a moment.
Filling a big bore
Let’s say your precharged pneumatic (PCP) is a big bore airgun. All big bores use a lot of air, and that has an impact on how they are filled. The big bores Dennis Quackenbush makes develop up to 500 foot-pounds and get two powerful shots per fill to 3,000 psi. They have relatively large reservoirs that accept a lot of compressed air. These guns can only be filled practically from an air tank. Imagine pumping a hand pump for 10 minutes to fill a gun, then firing just two shots before it needs to be filled all over again.
Now, it is possible to fill a Quackenbush big bore with a hand pump. I have done it. But if you intend shooting that airgun many times, then it is not practical to fill it that way. People will argue that a hand pump is perfect for hunters who want to get away from civilization, and they are right. A 5-pound hand pump that you can carry easily is certainly easier to handle than a 20-pound air tank that has to be refilled by either a fill station or an air compressor. However, if you are going to the range to sight in that big bore airgun at 100 yards and envision needing 20-25 shots to complete the task, then I don’t think you will want to use the hand pump — no matter how light and easy it is to carry. What you must ask yourself is which do you think you will do more often — shoot targets or hunt?
There’s still more to consider, and this is something most people aren’t aware of before they get into PCPs. A 3,000 psi 80-cubic foot aluminum scuba tank is relatively inexpensive. That doesn’t mean it will be cheap by any means, but a used one can often be bought for less than a hundred dollars. Compared to the cost of an 88 or 98 cubic foot carbon fiber tank that holds 20 times as much useful air (or more!), and can cost over $700, it does seem inexpensive. However — is it practical?
If you are filling a PCP to a pressure lower than 3,000 psi, a gun like the Benjamin Discovery that’s filled to 2,000 psi, for example, a 3,000 psi tank is great. You will get many full fills from one tankful of air, plus many more partial fills. However, if you are filling an airgun that requires a 3,000 psi fill you need to understand there is just one fill of air in that 3,000 psi tank. I don’t care how big it is, there is just one full fill. The next fill might end at 2,990 psi, or so. That’s because you are draining a tank that was filled to 3,000 psi to start with.
I understand that some dive shops will fill a 3,000 psi tank to 3,300 psi, but that’s because they know that the heat of compression drives the pressure higher. When the tank cools after the fill it will be very close to 3,000 psi. Some dive shops do this and others put the tank being filled in a large container of cool water. And some shops do both. The results are the same — 3,000 psi in your tank after it has been filled. That’s what you have to work with.
Think of it this way — a 10-gallon can holds 10 gallons of liquid. Pour out a teacup full and there are no longer 10 gallons in the can.
A carbon fiber tank that fills to 4,500 psi will fill a 3,000 psi airgun full many times — depending on the size of the tank. When the tank pressure drops to 3,000 psi there are still plenty of partial fills, so the net result is you get a lot more usable air out of one of these tanks. Of course they cost more, and are harder to get filled to their maximum. If you own just a Discovery one of these tanks might be an extravagance, but if you own a big bore airgun they are almost a requirement. With the testing I do I had to get two.
Filling a smallbore PCP
You might think that filling a smallbore PCP is the reverse of filling a big bore, and in some instances it is, but don’t assume that all smallbores are the same. A few are very sparing of the air they use, but many use lots of air because they are shot a lot more every time they are taken out. You have to try to envision what you will do with a smallbore air rifle. Let me give you an example.
I used to shoot a .22 caliber Career 707 a lot. It was a 10-shot lever action repeater that I shot anywhere from 100 to 500 times each time I took it out. In the beginning it was getting 70+ foot-pounds of energy for the first several shots and had maybe as many as 20 shots per fill. Not all the shots were that powerful but even the weakest was around 40 foot-pounds. Then I had the rifle highly modified and got around 90 shots per fill at a very consistent 30 foot-pounds. After the modifications is when I really started shooting the rifle a lot, because it had become so easy to shoot.
I made do with an 80 cubic-foot aluminum scuba tank, but that’s mostly because in the 1990s that was what we had. A few guys owned steel 120 cubic-foot tanks that were filled to 3,500 psi and they got a lot more shots than I did. But a large carbon fiber tank would still put that to shame!
The point I am making is that when you shoot a PCP you need high pressure air. Where you get it from is as important a decision as any other piece of essential equipment you will buy. This is one reason why I recommend starting out with certain PCPs that get filled to 2,000 psi. That gives you some time to become familiar with the technology so you can start asking questions that are pertinent.
Next I will address air compressors. These are used for two purposes. One is to fill a tank and the other is to fill the airgun directly. The smaller and less expensive ones are for filling airguns. Most of these are booster compressors.
A booster compressor such as the Air Venturi Power Booster takes air from a shop compressor that compresses it to around 100 psi and boosts it to 4,500 psi. They are slow to fill and need the input of a shop compressor to work. But they are the least expensive types of compressors. They cost between $600 and 1,000 new, but compared to stand-alone compressors, that’s inexpensive. What you save in money costs you in time and convenience. Some folks love them and others are turned off. I am among the latter group, only because I have two stand-alone compressors.
A stand-alone compressor does what the name implies. It is all that’s needed to compress air. Most of them today go up to 4,500 psi, but some stop at 3,000. You need to figure out which you want to use, because these start at $1,300 and go up over $3,000. I have to put in a plug for the compressor I believe is the best value right now and that is the Air Venturi compressor. It’s at the low end of the cost scale, yet it works very fast. I fill both my carbon fiber tanks from it.
The future of PCPs and their support equipment is very bright. I know of one stand-alone compressor from Sun Optics that’s due out very soon. From what I see, it will be a minor game changer, because it runs on either 110-volt household current or 12 volts from a car battery. I know of a couple hand pumps that are also in the works. The bottom line is, it’s going to be easier than ever to get compressed air in the near future. All it will take is money.