Methods of power adjustment — pneumatics: Part 2

by Tom Gaylord
Writing as B.B. Pelletier

Part 1

This report covers:

  • Pneumatics
  • Single stroke pneumatics
  • Multi-pump pneumatics
  • Precharged pneumatics
  • Short history of PCPs
  • Barrel length
  • Projectile weight
  • Barrel length and projectile weight together
  • Airflow
  • Springs
  • Valve stem travel
  • Valve angle and contact area
  • What’s the ideal?

This is the second part of a report on the methods of adjusting power in an airgun. Reader Riki asked for the report, and a number of other readers seconded his request. I wasn’t planning to also delve into CO2 guns, but several readers asked for that, and I will get to that in a different report. Today we look at pneumatics.

Pneumatics

A pneumatic airgun is one that uses compressed air to power the pellet. While a spring gun also uses compressed air, it is the method of compression that sets it apart from the pneumatics. In spring guns, a piston moves to compress the air at the instant of firing, where in pneumatics, the air is stored inside in a compressed state, waiting for the trigger to release some or all of it.

There are 3 fundamental types of pneumatic guns — a single stroke, a multi-pump and a precharged pneumatic. Each has its own methods of controlling the power and they must be examined separately. I will start with the single stroke.

Single stroke pneumatics

A single stroke pneumatic does just what the name says — it shoots a pellet using the air compressed from a single stroke. The pump head forms one side of the air reservoir, so if you try to pump it twice, the air that was compressed by the first pump is released.

Single strokes aren’t very powerful. All their design is focused on getting them to be as powerful as possible with one pump stroke. The only real way to increase their power is to increase the amount of air they compress. That can become an interesting exercise, but it boils down to increasing the swept volume of the piston. A wider pump tube or longer pump stroke are the only two ways I know of to do this.

Multi-pump pneumatics

Multi-pumps are pneumatics with built-in pumps that pressurize the air they store. They break down into two categories — those that exhaust all their air on one shot and those that get multiple shots from a fill. The multiple-shot types are mainly concerned with suniformity of each shot — not with how powerful they can be. If you want more power, you must be prepared to lose shot count or to compress the air to a higher pressure initially — or some combination of the two that makes sense. So, tuning the gun for power isn’t really the chief concern. However, what I am about to say about precharged pneumatics would also apply to multi-pumps.

Precharged pneumatics

Precharged pneumatics are guns that are filled with air from an external source and run on that air until the pressure falls too low. These are the oldest type of airguns known, unless you want to argue semantics and say that a blowgun is an airgun. Assuming you don’t, let’s now look at PCPs.

Short history of PCPs

We know that precharged airguns date back into the 16th century, probably to around 1550, or so. My book, BB Guns Remembered, has a short story about the invention of the outside lock airgun in the early 1700s. It’s speculative, but it’s based on an actual gun (receiver only) from about 1730 that I once had the opportunity to examine.

People were building PCPs five centuries ago, without the benefit of good seal material, efficient hand pumps and air compressors. How they did it is a wonderful topic for another report, but for today we will stick to the subject, which is power management in pneumatics.

Barrel length

If you have read my reports for at least the past year you know that barrel length is one major way a pneumatic gets more power. Just like with black powder, compressed air can do so much more when it has a longer time to push on a projectile. That being said, there needs to be enough compressed air available to use the longer barrel. A single stroke pneumatic can run out of useful barrel sooner than a PCP, because it has less air stored.

The AirForce Texan is the world’s more powerful production air rifle. At 500-plus foot-pounds, it is an efficient user of air because of its 33-inch barrel. When that barrel was cut to 24.75-inches for the TexanSS, 100 foot-pounds were lost.

I haven’t reviewed it for you yet, but the Umarex Hammer which is supposed to develop 700 foot-pounds at the muzzle will do so at the expense of an extremely long barrel. It’s so long that Umarex plans to shorten it by some amount to keep the rifle manageable. That rifle also has to use a special 500-grain cast lead bullet to achieve that power. Using the sabotted bullets we were shown at SHOT, the power has to be less, and cutting back the barrel will also reduce the power.

So, the length of the barrel is one key to velocity. Pay attention to some of the PCP bullpups you see and you will discover that, although they are shorter by design, their barrels are still quite long. To make an analogy many will understand, drag racers finish with higher speeds in quarter-mile races than they do when the race is only a eighth of a-mile long. The reason is easy to understand — a quarter-mile track gives them twice as long to accelerate.

A longer barrel is where a lower-pressure airgun like the Benjamin Maximus gets its power. Guns made centuries ago operated on less than 800 psi and were still powerful enough to kill big game at 100 yards. If you study them the one characteristic that stands out is their long barrels.

Projectile weight

The heavier the projectile, the greater the energy a pneumatic will generate. You must decide whether it’s energy you’re after or velocity, because in a pneumatic, one robs from the other.

I once saw a CO2 rifle called COToo Much that got better than 1,000 foot-pounds of muzzle energy back around the end of the 1990s. They did it with a 7.4 OUNCE (3,237.5 grains) lead projectile that looked for all the world like a flying doorknob. It only had to fly about 375 f.p.s. to do that and CO2 was enough to push it that fast. The gun was over 6 feet in length and weighed well over 15 lbs. It recoiled like an elephant rifle. So it wasn’t practical — it was a science experiment. Several airgun manufacturers have done similar things in the past and always the gun was impractical. Sometimes the barrel was super long and other times the projectile was way too heavy. When you deal with physics there are always practical limitations.

Barrel length and projectile weight

The two main factors in determining power and velocity and barrel length and projectile weight. After both of these the other factors are minor, though they are still significant.

Airflow

The compressed air has to flow out of the valve and into the barrel to power the gun. Anything that maximizes this flow is a potential way to increase the power — within limits. For example, a rifle that produces 35 foot-pounds might be boosted to 42 foot-pounds by opening the air transfer port. You can’t double the power this way like you can with a longer barrel or heavier projectile, but often these smaller increases are what you actually want.

To improve airflow several things must be addressed. First, don’t make the air turn too many corners. The straighter the flow the faster the flow and the more air will pass.

Next, the length of the air transfer port has some effect. Make it shorter for the best performance.

Next make the air passage as smooth and unobstructed as possible. Some PCPs have power adjusters that actually obstruct the passage of air by incremental amounts.

Springs

This topic relates to airflow, too, but I’m going to address it differently. The strength of the striker (hammer) spring and the weight of the striker/hammer help determine how long the exhaust valve remains open.

The strength of the exhaust valve return spring that pushes the valve head into the valve seat also determines how long the valve remains open. This spring is interdependent with the strength of the striker spring and the striker weight A delicate balance for all these factors needs to be found. This is the reason that air regulators work as well as they do, because these springs and weight can be adjusted for one pressure level at which they give optimum performance.

Valve stem travel

How far the valve stem travels with the valve head is also interdependent with the springs and the striker weight. The longer the valve stem travel, the longer the valve will remain open, allowing air to flow out. And the longer it remains open the greater the impact of adjusting the striker spring strength, which is another way PCP power is adjusted.

Valve angle and contact area

The angle of the valve has some impact on airflow, as does the surface area of contact of a valve. A smaller contact area produces results that are more precise (less velocity variation), which is why airgun guru John Bowkett has made valves with steel heads and seats. Their contact area could be razor thin, though they were extremely susceptible to dirt!

What’s the ideal?

Gee B.B., this is all well and good, but all I want to know is what is the best for all these variables? Fine — I will tell you that if you can tell me how long a piece of string is.

That doesn’t make any sense! String can be any length. There is no right answer.

Exactly, grasshopper!

45 thoughts on “Methods of power adjustment — pneumatics: Part 2

  1. BB,

    Thank you! I enjoyed that immensely! Though there was nothing “new” to me contained here, it is a very good idea to have such basics in a report together and to present such periodically so that “newbies” can have a better understanding of the fundamentals of this world they are beginning to explore. Bravo!


  2. Very nice. A good refresher and a couple of things that I had not heard of before. And to think, some people think all we do is, “Ohhh,…. you shoot bb guns”. Well,… there might be just a wee bit more to it than that.

    Good Day all,….. Chris


  3. Thanks a lot BB, I had no idea about pneumatics
    So the 12 fpe air rifles of Britain , the FAC ones of which produce 30 fpe, they are adjusted by changing the hammer springs and valve springs and hammer weight?
    .
    How does the trigger function in a pneumatic? My understanding is the action compresses the hammer spring , and it is held cocked by the sear. As the trigger is pressed , the hammer propelled by the hammer spring strikes the valve, lets out some air, and then the return spring closes the valve. Have I grasped it correctly?


    • Riki,

      If an air rifle model exists in 30 foot-pounds, then it cannot legally be reduced to 12 foot-pounds for the UK market. The Home Office assumes people could get the parts to convert it. So, although your assumption is correct, it never happens that way in the UK.

      Pneumatics have two different types of triggers — those that release the striker weight (knock open valve) and those that simply open the valve (blow open valve) What you said about the hammer weight and spring is the first type..

      BN.B.



  4. BB
    Thanks for a very informative report. The part about the amount of air transfered due to valve design in pneumatics, in my mind, is quite similar to how valve design and intake runners determine power in automotive engins. Different size valves but both are controling air flow.



  5. Michael, Great pics! Do you think those are punt guns that market hunters used on waterfowl before it be came illegal due to over harvesting? I’ve read about them but have never seen one. I think they were sometimes bolted into the bottom of a boat ,somehow, or braced against a block.


  6. Halfstep,

    Yes indeed, those are punt guns. The internet is loaded with pics of them, including pics of them strapped down to surprisingly small boats in ponds and small lakes. The recoil from a punt gun could easily propel a small boat for quite a distance. Hey, that would be a fine visual example for demonstrating Newton’s Third Law of Motion, I believe.

    Michael



    • Michael:

      As a boy, when I was growing up (1960s), our neighbor had a punt gun that had been in their family since the 1800s. He said that it dated to before the civil war. It was a double barrel, smooth bore muzzle loader with a barrel diameter of about 3″ each but not near as long as the one in the pictures. Total length was about 60″. I have no idea of how much black powder was used but he said the “shot” consisted of whatever was handy: buckshot, scrap metal, nails, etc. It was originally a flint lock but had been converted to percussion cap.

      According to his family history, it was originally mounted on a boat that hauled cargo up and down the Mississippi River and used to repel boarders. There would be one mounted fore and aft on the boat. Not sure that I can describe this correctly: The fore end of the gun was mounted to a “Y” bracket the let the gun move up and down with a shaft going down. The shaft stuck into a hole on the rail of the boat so the gun could be swiveled side to side. Basically it created a gimbal that allowed the gun to rotate both up and down and side to side. He said that as a boy (I’m guessing that this would be the 1910s or early 1920s) that he remembered the gun being used for market hunting.

      Their house burned down in the late 1970s and their punt gun was lost. Another piece of history gone.

      Jim


  7. @BB

    Very well worded article. I would say that you have covered everything that Multi Pump Pneumatic tuners have been doing for a long time (higher pressure [by better pistons and less head space], smoother air flow, larger transfer ports, lighter valve springs, heavier hammers, stronger hammer springs, and longer barrels), and that some PCP tuners have begun to do.

    Now if only the manufacturers of Multi Pump Pneumatic rifles would pay more attention, then we may see the ultimate small caliber pumper come to pass.


  8. A little off topic, but B.B., do you have any idea about why the velocity of my Condor SS is varying wildly? Every few shots the pellet will barely make it out of the barrel and fall on the ground in front of me. The shots that do make it on paper still hit high and low randomly. This is at 6 power. At 12, it seems to work fine, but that uses too much air for 15 yards!


    • Rambler,

      Yes. It’s obvious that you have just adjusted the gun too low for that particular gun. They are all different, but Condors do not do as well on the very low end.

      If you want less power and still want a 65 foot-pound gun you either need to use a different valve (a standard air tank or a MicroMeter tank) or a shorter barrel.

      B.B.


      • I haven’t adjusted it, except turning the power wheel, and I thought that it would still work fine at medium power. I have been using it with the co2 adapter valve and it seemed to be fine, and it was working well on air to begin with, but now it is giving me problems. Oh well. Thanks for the advice!


  9. B.B. ,have you ever done a review of the Daisy 777? Also, I read and enjoyed and was thoroughly edified by your reports on barrel twist impact on accuracy and velocity. When I asked you about twist rates earlier I was thinking about doing some informal testing myself just for giggles. After your exhaustive study I now know my time will be better spent finding something interesting to do with a cross section of tree trunk that I just drilled the center out of.
    🙂



      • B.B.
        Wouldn’t a 777 and a 747 be the same, save for the wood grips and “better” adj. sight? I know they both have the Lothar Walther barrel.
        Halfstep,
        /blog/2005/11/daisy-avanti-747-the-perfect-indoor-target-pistol/
        will be pretty close

        Doc



          • B.B. ,I read the blog Doc posted and wonder If you think Gamo Match pellets are as good in your 777s as you did the 747 you refered to. I have finally shot all mine up ( Mostly to break in guns as I haven’t found them to be accurate in anything I own) and don’t really want to buy more unless there’s a good chance they will be good in this gun.


        • I own 1 that probably hasn’t had 150 pellets through it and want to play with it again and hoped you’d be the man to tell me what to expect since, after all, you be da man. ( got caught up in Magnum Powered Spring Piston ,Must Go Fast,Mania soon after I bought it in early 80s and now I’m more mature about such matters) Was that a print article or something that I could get a link to?


        • Doc, if I may address you thus, I don’t know, but I do know my 717 was nothing to write home about. Fell free to call me Half. All my closest real life friends do. Not that you are imaginary ( I hope) . I mean the friends I can see.



  10. BB,
    “A smaller contact area produces results that are more precise (less velocity variation)”
    Will you please elaborate on this?

    You also said “…John Bowkett has made valves with steel heads and seats.”
    Why steel?






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