The Effect Of Caliber On Airgun Pellet Performance
Before moving on to discuss the effect of caliber on airgun pellet performance, it’s appropriate to remember several important conclusions from last month’s piece on pellet penetration:
1. Penetration is proportional to Sectional Density times Impact Velocity.
2. Pellet shape affects penetration, and pellets have a “flesh BC”, just like they do in air.
3. Pellet expansion drastically reduces penetration while creating a wider wound channel.
4. Wound cavity volume is relatively constant for a given FPE.
As we look at airgun pellet performance, let’s have a look at why larger calibers have an advantage when you take velocity out of the equation. Since modern PCPs can reach into the 900s in any caliber, this is a fair thing to do.
Sectional Density Increases With Caliber
For a given shape and proportions, the Sectional Density (SD) is proportional to the caliber. This is most clearly demonstrated with roundball. Check out the chart below:
The chart above is for lead spheres. While the weight increases with the cube of the caliber, since the cross-sectional area increases by the square, the SD is proportional to the caliber.
This means that if the velocity is held constant, the penetration would increase in proportion to the caliber.
That is only part of the story, however. Since the weight of the roundball increases with the cube of the caliber, if the velocity is constant, the FPE increases with the cube of the caliber as well.
Since wound channel volume is roughly proportional to the FPE, this leaves little doubt why larger calibers have so much more killing power.
Let’s compare a .25 cal roundball to a .50 cal one, both traveling at 950 fps. The .25 cal roundball weighs 23.4 gr. and has 47 FPE of energy, while the .50 roundball weighs 187 gr. and has 375 FPE of energy. That’s 8 times as much just by doubling the caliber!
The wound channel for the .50 cal should have 8 times the volume, and here is the reason. The hole it drills is 4 times the area (because that increases by the diameter squared), and twice as deep (since penetration is velocity times SD). There is the mathematical reasoning behind the wound channel volume being proportional to the FPE.
Now of course we don’t normally shoot roundball in airguns, we shoot pellets or slugs. However, the same principles apply.
Larger calibers tend to have heavier pellets, and those pellets, for the same shape, have a higher SD. Here is a chart showing the range of typical pellet weights for various calibers, and the effect of both on the SD.
There is, of course, a wide range of pellet weights available within each caliber. A heavy pellet in one caliber will have a higher SD than the same pellet weight in a larger caliber.
However, it is harder to accelerate that pellet in the smaller caliber, so the advantage in SD tends to shift to the larger calibers, just like with roundball. Since ultimately the FPE of a gun is proportional to the bore area (caliber squared), there is no question that “killing power” goes up with caliber.
Meplats and Hollowpoints
OK, so we know that round nosed pellets penetrate further for a given SD and velocity. However, this can mean that they “icepick” right through your quarry, and waste some of their energy by doing that.
Is there any way that a pellet can be made to lose more energy inside the target? Well of course there is, you can give it a Hollowpoint that causes it to expand. There is, however, another way. You can create a flat on the front of the pellet, called a Meplat.
I did exactly that with some 50 gr. JSB Exact .30 cal pellets, and then shot them at 100 FPE into a soap block. All I did was sand off some of the round nose to create a Meplat of 50% of the caliber (ie 0.150”), which removed about 1 gr. of weight. I also made a hollow point version by using a “center drill” to create a cavity in the nose, removing about 2 gr. of weight. Here are the results of them hitting that soap block:
Obviously, the Hollowpoint made a huge difference, compared to the original round nose pellet. It penetrated only about half as far. However, the round nose pellet with the Meplat also didn’t penetrate as far, and it left a slightly larger diameter wound cavity than the round nose pellet. Here are the recovered pellets:
The amount of extra energy that a pellet or slug with a Meplat sheds inside your quarry depends on the diameter of it, and to some extent on the shape of the nose behind it.
The slight flat I created on that 50 gr. Round nose pellet didn’t change the shape a lot. However, it hit hard enough to shorten the pellet visibly. It didn’t reduce the penetration or increase the diameter of the wound channel by much, but a larger flat would do both.
For those of you who wish to cast your own pellets, NOE Bullet Moulds (https://noebulletmolds.com/site/shop) now makes molds for .22, .25, .30 and .35 cal pellets, in varying weights and styles. Here is an example of their .30 cal “Magnum Hunter” design:
They come with 4 different base pins so that you can change the weight to suit your gun. The ones above range from 58-72 grains. These round nose pellets with a 60% Meplat have developed a reputation for hitting very hard.
Use “Enough” Gun
There is no better way to wound an animal or bird than by not using a powerful enough gun for the job. In Canada, if you don’t have a “Possession and Acquisition License” (PAL), you are limited to airguns that shoot less than 500 fps. Just to show you how little power that really is, here is a soap block that I shot at with various pellets from a “non-PAL” .22 cal air rifle.
There were even four Hollowpoint pellets used in that test. None of them showed even a hint of expansion. The impact energy was about 5 FPE, to simulate how a typical pellet from a non-PAL gun would hit at 25 yards. Compare these impacts to the ones last month at 20 FPE. Huge difference.
Now look at the impact of the .30 cal pellets at 100 FPE in the top photo above. Compare that to the 20 FPE impacts from last month. Again, there is no comparison, the .30 cal wound channels are huge. In fairness to your quarry, PLEASE use enough power to do the job humanely. Airgun pellet performance is important!
Next month I will start talking about the Terminal Ballistics of slugs.