What pressure is best for a PCP?

by B.B. Pelletier

Last week, a reader named Dave asked, “What is the lowest pressure that will produce acceptable results in a PCP? How many fills will a scuba tank be able to give, given that low operating pressure?

Dave, you happened to hit on one of my favorite subjects in airgunning – efficiency!

How low can you go?
I have been saying this for a long time – barrel length is an important part of precharged efficiency – maybe the most important part! On the Mythbusters TV show, they shot a 4-lb. chicken carcass from an air cannon at 130 m.p.h. on just 25 psi! When they bumped the pressure up over 100 psi, four times as much, the velocity increased by less than 5 m.p.h. So, 25 psi was all it took to accelerate a 4-lb. projectile to 190.7 f.p.s. when shot from an 8′ cannon barrel.

Airguns from history got high velocity from very low pressures. Typically, they were pressurized to 500 psi, yet had the power to launch a .50 caliber lead ball with enough force to kill deer-sized game beyond 100 yards. The recorded velocities of the vintage big bores are in the 500 to 600 f.p.s. range. It wasn’t just long barrels that produced such remarkable results – they also had air valves that remained open far longer than modern PCP valves do.

In Airgun Revue No. 5, Tom Gaylord wrote about a Gary Barnes’ big bore rifle that shot a .457 caliber lead ball at 800 f.p.s. on just 750 psi! That rifle had 10 good shots at that pressure! The barrel was over 32″ long, which is long but necessary for low pressure to accelerate a projectile to high velocity. Now, Dave, how many HUNDREDS of fills would that rifle get from an 80 cu.-ft. 3,000 psi scuba tank? A heck of a lot more than a rifle that needed 3,000 psi, that’s for sure.

Something more practical
Many airgunners know about the field target rifle Mac-1 is selling. It was designed by Larry Durham to operate on less than 2,000 psi and gets over 50 full-power shots (in .177 ,that’s a JSB Exact going around 900 f.p.s.) with very tight velocity variation. At the recent airgun show in Little Rock, Durham sold a .22 sporter version of this same rifle. It develops about 20 foot-pounds per shot and gets 55 shots. It fills to 1,650 psi and shoots down to a low of 1,250, so all 55 shots are fired on just 400 pounds of air!


This Mac-1 Hunter is a 20 foot-pound .22 rifle that gets 55 consistent shots on just 400 psi of air.

A PCP rifle like that is so easy to fill that you could do it with a hand pump, using just one hand! Or that scuba tank that needs a refill because it only has 2,200 psi remaining suddenly becomes a storehouse of air! Unfortunately, the Mac-1 Hunter, as it is known, is very close to handmade and carries a price tag to reflect the time it takes to produce.

AirForce built a special valve to operate on just 1,500 psi. It was made to use indoors where power isn’t needed but accuracy is. They got over 100 good shots with .177 pellets going about 475 f.p.s. Best of all, there was very little variation from the first shot to the last. It was an offshoot of the new MicroMeter valve.

So, high pressure isn’t the only way to run a PCP. A gun can be made to shoot well on pressure so low it becomes very cost-effective.

13 thoughts on “What pressure is best for a PCP?

  1. Some strange facts there,you know,I knew that it was alot about the barrel not just high pressure but alot of all that I never knew.


  2. Jed,

    I neglected to say that the AirForce gun that used their experimental valve only had a 12-inch barrel! They probably would have gotten velocities in the mid-500s with an 18-inch barrel.

    B.B.


  3. BB,

    Your mention of a 32″ barrel piqued my curiosity. Now, a wild guess will do here…. Assume 45 ftlbs from a 24″ barrel, as a starting point. At what theoretical barrel length would you guess velocity stops increasing before friction and other forces take over? Just looking for ballpark, e.g. is it 48″ or 120″ ?

    Will


  4. In a previous article talking about how safe PCP is, you mentioned hearing about someone making his own 3,000 p.s.i. cylinder and testing it foolishly with air instead of hydraulic oil. I intend to build my own PCP cylinder and would like to know what other advice you might have. Also, running around roughly 2,000 p.s.i., would threaded and sealed ends be sufficient for the cylinder, or would welding be required? Just so you know, any materials, such as the tubing itself, would be purchased from a manufacturer who actually rates their product for the pressures required, so no worries about shoddy materials. Better empy pockets than no hands to reach into them with.
    Also, I was thinking of the potential to convert my 1377 to PCP. Do you know what p.s.i. the valve is used to operating at under ten pumps?


  5. A TEST WAS DONE IN TH 1970′S BY A FIREARMS WRITER. HE MOUNTED A 16 INCH BARREL ON A COLT 1911A1 IN .45ACP. CRONOGRAPHING AFTER ONE HALF INCH AT A TIME WAS REMOVED, HE FOUND THE VELOCITY INCREASED UNTIL 10.5 INCHES WAS REACHED, THEN BEGAN TO DECREASE.10.5 INCHES WAS THE NONINAL BARREL LENTH THAT THE THOMPSON MIAI CARRIED. I WONDER HOW THE DESIGN TEAM DISCOVERED THAT. CALCULATION? ACCIDENT? OR TESTING? YOU CAN’T BE SURE OF HOW ALL THE COUNTERACTING FORCES INSIDE A PROJECTILE DEVICE WILL WORK WITH MATH. TEST.
    FLINTLOCK


  6. Thanks for answering my question. I’ve got a tangential question. You have written about barrel length and its effect on velocity before, and about CO2 guns preferring heavier pellets.
    So, given a 3” barrel (like the Walther CP99) and its CO2 power, the literature says the velocity is 360fps. The unknown here is the pellet weight. I assume that a good bit of CO2 is wasted with each shot due to the barrel length. If heavier pellets are used, will the velocity remain the same, or drop only slightly, as there would be less wasted gas due to the need to move the heavier pellet out of the barrel. Are there any rules or hints to help selecting pellets for this type of gun?
    Thanks again, Dave


  7. Will,

    I have wondered that myself. I look to blackpowder for the answers, because those guns seem to perform so similarly. At some point, black powder stops burning in the barrel and you don’t get more velocity from a longer barrel. I know of a few black powder long guns with barrels greater than 72 inches, so I suspect the limit is around there, depending to some extent on bore diameter.

    With a smallbore air rifle, I would guess the theoretical limit is aroung 48 inches or so. It might be fun to test, but nobody makes a barrel that long in such a small caliber, so it’s a moot point at this time.

    B.B.


  8. airforce45,

    The 1377 probably operates around 1,000 psi or so. In the book Gas, Air and Spring Guns of the World, they did a test (in 1957) in which they got a multi-pump rifle up to 1,500 pai. Dennis Quackenbush repeated this test a few years ago and hit 1,200.

    Use seamless hydraulic tubing with a burst rating four times your working pressure. Yes, threading is acceptable, but follow good machining guidlines. And remember that cutting threads into a tube reduces its rating, so make all caculations based on the thinnest diameter in your design.

    B.B.


  9. Dave,

    I think YOU need a chronograph! You are interested in the things that only a chronograph reveals.

    In my experience, only one time was a lighter pellet slower than a heavier pellet in a given gun. Heavy pellets are almost always slower.

    Don’t think about velocity – think energy, instead. If you do, you will soon start understanding the way pellet guns work. And you will be able to predict how each gun will perform under certain circumstances.

    B.B.


  10. The Mac-1 Hunter doesn’t shoot on just 400 psi of air – your figures give a minimum pressure of 1250. Without knowing the volume of the tank, the 400 psi drop is meaningless.

    Not the best wording for a posting on lowest reasonable pressure for a PCP. Why not point out that the lowest pressure (modern) pre-charged guns are effectively the co2s, running in the low 800s?


  11. I agree that I was using the slang term when I said 400 pounds of air, but since it is used universally throughout the airgun community, I figured no harm.

    You are correct when you say that I told you nothing about the volume of air consumed – HOWEVER – and this is what really matters – since the max fill is only 1650, you CAN fill that rifle with a hand pump, using only one hand. The thing I neglected to specify was how many pumps strokes that would be.

    B.B.


  12. I dont really know if this question really belongs here (Couldn´t find any other place to post this), but since were talking high pressure and tanks i just have to ask.
    If i´m short on scuba tanks but have access to an awful lot of nitrogen tanks, is it possible to fill a PCP with that (Regulated down to 3000 psi) AND being able to use the rifle? Or will it compromise safety, accuracy, velocity, durability or something?


  13. mathias,

    That question gets asked all the time because industrial gas suppliers must seem less threatening to airgunners than dive shops.

    Nitogen is the major component in air and it is not corrosive, so it shouldn’t do any damage to the seals in an air tank or gun.

    On the other hand, most airgun manufacturers speficy the use of air, alone, so if it can be proven that nitrogen was used, the warranty, if any, will be voided.

    B.B.


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