Power and pellet weights

by B.B. Pelletier

This report is another response to a viewer of our Airgun Academy videos. In episode 22, we say the following at 3 minutes, 20 seconds into the video, “When using real hunting pellets, you have to realize that the velocity and, therefore, the power is going to be significantly less [than the light pellets the rifle is advertised to shoot fastest].” A viewer took issue with that statement, so today I’d like to explore how airguns handle pellets of different weights.

There are three different types of airgun powerplants: pneumatics that store air under pressure and release it with the shot. This compressed air pushes the pellet and gives it it’s power. The pneumatic powerplant pushes the greatest volume of compressed air behind the pellet and, depending on design considerations, is potentially the most powerful type of airgun powerplant.

Spring-piston airguns store no air. They have a spring-powered piston that releases with the shot and moves forward to compress a very small amount of air that gets behind the pellet to push it. The pressure of this compressed air is very high, but the volume is very small; once the pellet starts down the barrel, the air pressure behind it drops off fast. By the time the pellet leaves the barrel, there’s very little pressure in the air behind it — especially compared to a pneumatic airgun.

Guns that use carbon dioxide act more like pneumatic guns, except that carbon dioxide is under less pressure than compressed air; plus, it expands slower because its molecule is larger than the atoms contained in compressed air. CO2 guns act like pneumatics to a point, and then they’re limited by their use of the larger CO2 molecule, where compressed-air guns, which are pneumatics, have much higher limits.

How it works
How does this affect the performance of an airgun? Most commonly, when the pellet weight increases. The power of a spring-piston gun decreases, and, of course, the reverse is also true. It’s not an absolute physical law, but only a general relationship. There are some design considerations such as the contact surface of the pellet with the bore and the lubricity of the lead alloy that can change this relationship slightly. However, the relationship still stands.

British airgun magazines have been talking about this since the 1980s. It’s very important to them because of their legal 12 foot-pound power limit. If a new pellet can come on the market and increase the performance of certain airguns that are currently legal so they exceed the legal limit of 12 foot-pounds, then the entire airgun community needs to be aware of it! Once it becomes known that a certain pellet can do that, the authorities will be using that pellet to test all airguns. Let’s put this relationship to the test today and see if it holds any water.

Using a .22-caliber Diana 27 spring rifle, I’ll shoot three different weights of pellets. If the relationship holds true, the lightest-weight pellet should produce the greatest power, the medium-weight pellet should produce the second-greatest power and the heaviest-pellet should produce the lowest power.

RWS Hobby
The .22-caliber RWS Hobby pellet weighs 11.9 grains and averages 490 f.p.s. in the Diana 27. That means it produces an average 6.35 foot-pounds of muzzle energy.

Crosman Premier
The .22-caliber Crosman Premier pellet weighs 14.3-grains and averages 459 f.p.s. in the Diana 27. It produces an average 6.69 foot-pounds of energy at the muzzle.

Beeman Kodiak
The .22-caliber Beeman Kodiak pellet weighs 21 grains and averages 352 f.p.s. from the Diana 27. It produces an average 5.78 foot-pounds of energy at the muzzle. I am aware that the Pyramyd Air website says the Kodiak weighs 21.14 grains; but the Kodiaks I’m using are several years old, and I’ve weighed them on an electronic scale at exactly 21 grains.

So, we already have an exception to the general rule, with the Premiers producing greater muzzle energy than the lighter Hobbys, where the relationship predicted the opposite. But the general trend does remain in force, as the much heavier Beeman Kodiaks produce significantly less muzzle energy than the lighter pellets.

Now let’s try these same three pellets in a tuned Beeman R1 and see what happens. If the relationship holds, we should see the lightest pellet making the greatest energy and the heaviest pellet the least, in a linear relationship.

RWS Hobby
The .22-caliber RWS Hobby pellet averages 817 f.p.s. in the R1. That means it produces an average 17.64 foot-pounds of muzzle energy.

Crosman Premier
The .22-caliber Crosman Premier pellet averages 750 f.p.s. in the R1. It produces an average 17.87 foot-pounds of energy at the muzzle.

Beeman Kodiak
The .22-caliber Beeman Kodiak pellet averages 575 f.p.s. from the R1. It produces an average 15.42 foot-pounds of energy at the muzzle.

Again, the Premier pellet stepped out of line by producing the greatest energy. But the Kodiak maintained the relationship.

What does this prove?
It doesn’t prove anything. It demonstrates a general relationship between pellet weight and power in a spring-piston airgun. You could test 10 more guns and get several more anomalies, including a gun that actually shot the heaviest pellet with the greatest power. In fact, I’ll tell you how to do that in a moment.

But if you tested 10 different spring-piston air rifles, you would probably still see the general relationship holding most of the time. I’ve been doing this for many years, and I’ve seen it happen too many times to doubt that the relationship does work as described.

How to beat the relationship
I learned, when testing several exotic tunes while writing the Beeman R1 book, that a heavy piston always favors the heavier pellet. So, simply adding sufficient weight to a piston will change everything. But it will also give you more piston bounce and poor performance with a broader range of middleweight and lightweight pellets — which is why the pistons of spring guns weigh what they do. They’re made to give the broadest possible range of performance within the expected power band of the rifle they were made for.

When I wrote the script for episode 22, I was thinking of spring-piston airguns when I wrote the line that the viewer took exception to. That’s because the huge preponderance of airgun hunters today use spring-piston rifles.

Before you jump down my throat for saying that, I do realize that there are thousands of hunters using PCPs; and in some warm spots, there are even hunters with CO2 guns. But that doesn’t change the fact that most airgun hunters in the U.S. still use spring-piston rifles today. I shouldn’t have made a broad statement like that in the video without qualifying it, and the viewer was right to voice his concern. We’ve added corrective text to the video at that point.

But this report isn’t really about that video. It’s about learning how pellet weight performs in an airgun. According to this logic, precharged guns develop more energy with heavier pellets and less with lighter pellets. So, let’s switch over to a precharged pneumatic rifle and run the same three pellets, to see what happens. If the relationship holds as it’s stated, the heaviest pellet should be the most powerful and the lightest the least powerful.

For this test, I used an AirForce Talon SS with an optional 24-inch .22-caliber barrel. The power was set to 10.

RWS Hobby
The .22-caliber RWS Hobby pellet averages 1035 f.p.s. in the SS. That means it produces an average 28.31 foot-pounds of muzzle energy.

Crosman Premier
The .22-caliber Crosman Premier pellet averages 982 f.p.s. in the SS. It produces an average 30.63 foot-pounds of energy at the muzzle.

Beeman Kodiak
The .22-caliber Beeman Kodiak pellet averages 882 f.p.s. from the SS. It produces an average 36.28 foot-pounds of energy at the muzzle.

So, this time, the relationship held exactly as predicted. You can expect the same relationship to play out in every pneumatic, regardless of the power level at which it performs.

So, what?
Okay, I’ve explained an old relationship between pellet weight and performance. What about it?

A couple of things, actually. First, with the modern uber-magnum spring rifles, you can expect to see a lot of reversals in the relationship. That’s because they have heavier powerplants that are designed for heavier pellets. So, things may not be as cut-and-dried as you see here.

Second, I want those of you with chronographs to do your own tests and report the findings. That way, we’ll see if the relationship still holds over a much wider sample of airguns and pellets than what I’ve shown. Just choose pellets with weights that are separated by a good margin, so each one stands apart from the others.

And, finally, this is a lesson you need to internalize, because it’s fundamental — or at least I hope that all of us can prove that it still is. In the same way that a longer barrel increases the velocity and power in a pneumatic, this relationship will help you as you move forward in your airgun journeys.

Air Arms S400 MPR FT: Part 4

by B.B. Pelletier

Part 1
Part 2
Part 3

The Air Arms S400 MPR FT rifle still has a surprise for us!

This special report about the Air Arms S400 MPR FT rifle was unplanned, but blog member Coax asked for it. Today will serve as the best lesson I’ve ever written on how to properly use a chronograph, because I made a huge mistake and the chronograph straightened it out for me.

Coax told me about a transfer port limiting screw that could be turned out to increase the velocity of the rifle. I was unable to locate it from his description, and we went back and forth several times before I found it. At least, I thought I’d found it. Therein lies the huge mistake I made, and the save made by the chronograph, all of which should be a good lesson in pneumanology.

The “secret screw”
Below is a photo of where the power-adjustment screw resides. But don’t just loosen the screw in that picture or you will be guilty of the same huge mistake I made. Because I did loosen it 2.5 turns and I got results. They were quite positive and I was already writing today’s report in my mind, after recording each of 99 shots, when I discovered a huge mistake. I will share those results with you now, but please don’t act on them until you’ve read this entire report, because that screw isn’t the one to adjust this rifle!

After much communication, I finally located the “secret screw” that Coax is talking about. But I got a huge surprise, so please read the entire report.

I filled the reservoir to an indicated 190 bar. I read the scale on my AirHog carbon fiber tank, which I know to be reasonably accurate. Then, I began shooting Crosman Premier lites, which were featured in Part 2 of this report.

Shot Vel.
1….. 784
2….. 791
3….. 783 (lowest velocity recorded)
4….. 785
5….. 792
6….. 790
7….. 785
8….. 790
9….. 784

At this point, I’ll reflect on what we’re seeing, even though it’s not the result I was after, nor had I done the right thing yet. If you go back to Part 2, you’ll see that when I filled the rifle initially, I filled it to 190 bar on the tank gauge. The manometer on the rifle read lower than that — about 180. The initial velocity was in the 764 f.p.s. range with the same Premier lites that were used in this test, so turning out the small screw on the right side of the receiver seemed like the right thing to do. Because, as you can clearly see, the rifle started out 20 f.p.s. faster and averaged about 791-792 f.p.s. for the first 30 shots. The maximum velocity spread was 17 f.p.s. for this 30-shot string.

So I continued.

Shot Vel.
35…..804 (about 170 bar)
48…..814 (fastest shot)
54…..802 (150 bar)

Okay, we’re up to 60 shots on this fill and no sign of the power dropping. Also, we’ve dropped below the 150 bar pressure level, according to the onboard pressure gauge (manometer). In this string of 30 shots, that is from shot 31 to shot 60, the average velocity has climbed to just about 800 f.p.s. That’s 16 f.p.s. faster than the average of the first 30 shots, so the gun’s increasing in power. The maximum velocity spread for this 30-shot string is 18 f.p.s.

Let’s continue.

Shot Vel.
87…..792 (125 bar)

The final string shows the end of the power curve and the rapid drop-off back to the lowest velocity recorded in the beginning. Now I’ll analyze the entire string of 99 shots as I would see them if I were using the rifle to compete in field target.

The rifle really came up on the power curve at around 170 bar indicated on the rifle’s manometer. That was at shot 35. I would fill to that level, after making a permanent mark on the gauge, so I could find that level easily when filling again. If I consider shot 84 to be the final good shot in the gun, I would have from shot 35 to shot 84 as a useful string. That’s 49 good shots, which I would round up to 50 shots. I would have 50 good shots in the gun that went as slow as 795 f.p.s. and as fast as 814, for a total shot string variance of 19 f.p.s. That’s pretty good; and if you check back with Part 2, you’ll see that I’ve actually raised the average velocity of the rifle by about 15 f.p.s. over the useful string. So, adjusting the “secret screw” did change the performance of the rifle.

Only that wasn’t the secret screw! After completing this exhausting test and evaluation, I was wondering why Coax said the secret screw was INSIDE a deep threaded hole. Clearly it wasn’t on my rifle. Unless…!

Oh, my, gosh! I ran that whole test and never even touched the real secret screw! That screw, which Coax apparently is missing, is only the cover for the real screw. That was the huge error I made.

The real transfer port adjustment screw is located deep inside the hole that remains when the cover screw shown at the top of this report is removed. The Allen wrench is a 0.050-inch size.

Saved by the chronograph!
Here’s the real lesson for today. Because I had that beautiful, pressure-related velocity curve indicated in those 99 shots shown above, I didn’t need to waste any time once I adjusted the real screw. I filled the rifle to only 170 bar as indicated on the onboard manometer and started the second test.

Then, I turned the real power adjustment screw out one and one-half turns and shot a 10-shot string.

Shot Vel.
Avg. 886 f.p.s.

Next, the screw was turned out 1 additional turn.

Shot Vel.
Avg. 893 f.p.s.

The screw was turned out one additional turn.

Shot Vel.
Avg. 894 f.p.s.

What I learned
First, I learned that this was indeed the true power adjustment screw. Second, I learned that turning it out about two full turns is all that’s necessary. After that, the velocity increases are not significant. I finished the 30-shot series with about 140 bar left in the reservoir, so there are about 15 more good shots in the gun.

Taking the second string average as a power input number, the rifle now generates exactly 13.99, which is close enough to 14 foot-pounds of muzzle energy with this pellet. Use a heavier pellet and get more power because this is a pneumatic.

By referring to the chrono data chart that also had the pressure indicated, I didn’t need to waste any time running up to power. I knew where the power band was located, even when I was increasing the airflow. The valve still works the same, regardless.

That’s the power limit of this rifle. I imagine I could get up to 16 foot-pounds if I used a very heavy pellet, but the best pellet is always the most accurate one. Whatever that one produces is the practical maximum for this rifle.