Methods of power adjustment — pneumatics: Part 2
by Tom Gaylord
Writing as B.B. Pelletier
This report covers:
- Single stroke pneumatics
- Multi-pump pneumatics
- Precharged pneumatics
- Short history of PCPs
- Barrel length
- Projectile weight
- Barrel length and projectile weight together
- 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.
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-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 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.
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.
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.
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.
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.