Airgun Pellet Penetration – The Real Story
This month, HAM Technical Editor Bob Sterne talks about airgun pellet penetration. In this article he will give some answers about why pellets perform the way they do. Airgun hunters – in particular – will find this an interesting discussion!
After our pellet or slug reaches the target, assuming you are not just shooting at a paper target, we are interested in how that pellet performs.
Does it penetrate far enough to reach vital organs? Does it pass through completely? Does it expand and deliver more of its energy to the target?
Penetration of a non-expanding projectile of a given shape (eg. a sphere) is proportional to the Momentum (not the FPE) divided by the cross-sectional area.
Momentum is mass (weight) times the velocity on impact.
A light pellet traveling faster can have the same momentum as a heavy pellet traveling slowly. If they are both the same caliber and shape, they will have approximately the same penetration.
If you compare a small caliber pellet to a larger one, and they have the same momentum, the smaller one, which has a smaller cross-sectional area will see a smaller force slowing it down as it penetrates, so it will penetrate further. The momentum dissipates more slowly.
This makes it a bit difficult to compare airgun pellet penetration between calibers. Fortunately, there is a simple answer for that…
Sectional Density times Velocity
We often use the term Sectional Density (SD) when referring to projectiles. It is a way of comparing pellets of different calibers. To calculate it, we use a simplified formula (PI/4 is omitted when calculating the Ballistic SD ):
SD = Weight / 7000 /Caliber ²
The weight is in grains, and the caliber is in inches. We square the caliber to change the equation to represent the area, instead of the diameter.
If we multiply the SD by the velocity (in fps) on impact, we get a “penetration number” that can be used to compare different projectiles.
Here is the formula:
Penetration is proportional to SD x Velocity
Double the SD, you will double the penetration for the same velocity. For a given SD, double the velocity you will double the penetration.
The actual depth of penetration will depend on the media (material) impacted, and the shape of the projectile.
Over the years I have tried many different media for measuring airgun pellet penetration and expansion of projectiles.
Wood is too hard and inconsistent, wax is too hard, ballistics gel doesn’t hold the cavity shape well and is temperature sensitive and expensive. Water doesn’t hold the cavity at all, and is “thinner” than tissue. Putty is both temperature sensitive and in my opinion shows wound cavities much larger than realistic.
So, I have settled on “melt-and-pour” soap as my standard material.
This soap is consistent, not temperature sensitive, not overly expensive, and can be melted and reused. It can be cast into any size and shape, and melts in a pot suspended in boiling water.
It is clear enough to photograph the wound cavity, which holds its shape well. Furthermore, you can easily measure the depth of airgun pellet penetration. In addition, you can dissect the wound cavity and measure it if desired, and recover the projectile and examine it afterwards.
It is my opinion that it is a fairly good representation of flesh, as a .22LR High-Velocity 40 gr. RN rimfire bullet penetrates about 11” and the 37 gr. HP version half that.
Editor’s Note. Hard Air Magazine also uses soap for the impact section of our pellet test reviews. So we’re on the same page here.
Over the years I have tested the penetration and expansion of many pellets. If they expand and change shape, they don’t follow the simple penetration rule above. But for ones that don’t expand, the correlation is remarkably good.
The dotted line is what we would expect, a linear relationship between the product of the SD and velocity, to the airgun pellet penetration.
The large dots represent data I have collected, and you can certainly see that they tend to follow the predicted trend.
Remember that this data includes all non-expanding pellets I have tested over many years. It includes round nose, pointed, and domed pellets of various shapes, and even round ball. Not much wonder it isn’t a perfectly straight line when it includes such a variety of shapes!
A Different Kind of Ballistics Coefficient
You are by now familiar with the concept of the Ballistics Coefficient (BC). It is a measure of how quickly a given pellet shape slows down in air. Does it not make sense that the same idea might be valid in flesh?
It seems perfectly reasonable that a wadcutter of the same weight would slow down quicker in flesh (or soap) than a pointed or round nosed pellet. The SD and impact velocity might be the same, but you would expect – and in fact we do see – less penetration for pellets which have more drag in air.
Throughout this article you will see four photos of pellet impact in soap. The pellets were shot from a 20 FPE .22 cal PCP air rifle.
In these photographs, you will notice several interesting and important trends.
1. Heavier pellets penetrate further than lighter ones. Pay particular attention to the left side of this photo, below. Those are all JSB Exacts, of different weights.
Although they were all shot at the same 20 FPE (heavier pellets will be only a bit more), heavier pellets have more Momentum for the same FPE. That is because when calculating the FPE, the velocity is squared, so it has more effect on the FPE than on the Momentum.
Wadcutters display less airgun pellet penetration, and sometimes they actually shorten because the drag of passing through the soap is so great.
2. Round nosed pellets have the greatest penetration, even greater than pointed pellets of the same weight. They have a better BC in air than pointed pellets, apparently the same thing happens in soap!
3. Pellets with a shallow dome (eg. RWS Superdome) have a bit less penetration than a true round nose design (JSB Exacts, Crosman Premiers, H&N Baracudas).
4. Lastly, Hollowpoint pellets that expand have the least penetration of all, and the wound cavities are much wider.
The Wadcutter and Hollowpoint pellets from the above test were recovered, and are shown below:
I checked all the other pellets, and none showed any noticeable deformation. The distortion of the pellets in the above photo indicates energy expended inside the soap. (That also applies to the photos in individual HAM pellet reviews). This distortion shows up as extra width in the wound channel, instead of penetration.
Although tough to prove, it appears that the wound channel volumes are all quite similar (because they were shot at the same FPE). The deepest penetrating pellets have the narrowest wound channels.
Hollowpoints that expand have shorter but wider wound channels. This is consistent with the other widely accepted “fact” about wound channels. They are roughly proportional in volume to the FPE!
I will continue the concepts of airgun pellet penetration next month. Stay tuned!