Airgun Slug Penetration Compared to Pellets

In the previous two articles I dealt with the penetration and expansion of pellets. Now let’s look at airgun slug penetration and compare the two types of ammo.

While there are some pellets that are designed to expand, for the most part they do not have a very good Ballistics Coefficient (BC). This limits their effective range, so most hunters use round nosed pellets.

These tend to “icepick” right through game, and so require a very precise shot to drop an animal in its tracks. Also they end-up wasting a lot of their energy through over-penetration. Fortunately, many round nosed pellets are also the most accurate, and so they allow that precise shot placement.

Slugs, on the other hand, often use a Hollow Point (HP) design to limit penetration and deliver more energy to the game via their expansion. Below we see a photo showing a comparison between a couple of pellets and slugs. All are .22 cal and were shot at 950 FPS, so the penetration in the block of soap should be proportional to their weight.

Note how much further the 34 gr. JSB Beast pellet penetrated than the 18 gr. JSB Heavy, as expected.

We might have expected the 30 gr. flat nose slug to penetrate almost as far as the 34 gr. pellet, but instead it was comparable to the 18 gr. because the flat nose caused it to expand slightly and create a wider wound channel.

That effect is dramatically increased with the 27 gr. hollow point slug, which penetrated only half as much as its flat-nosed brother, while leaving a gaping wound channel.

Slugs Perform Differently Than Pellets

Here is the block of soap cut open to reveal the wound channels of the 18 gr. pellet and the 30 gr. flat nose slug.

Although the penetration was similar, the 30 gr. slug created a dramatically wider wound channel (more volume), which is exactly what we would expect, because although the velocity was the same, the FPE of the slug was 66% greater because of the increased mass.

Below is a photo of the 34 gr. JSB Beast embedded in the soap.

You will note that both JSB pellets have virtually no distortion or expansion, despite an impact velocity of 950 FPS. Now let’s look at what happened to the 27 gr. hollow point (HP) version of the slug.

Wow, what a difference! At 950 fps the HP version of this Bob’s Boattail NOE slug came apart. It created a huge wound channel, leaving fragments behind, and penetrating only half as far as the flat nosed slug it is based on.

Recovered Pellets and Slugs

As we have seen before, round-nosed pellets hardly expand at all. The same would happen if a slug was a round nosed design, it would just penetrate right on through most game.

However, airgun slugs are for the most part designed to hit hard, and almost all slugs that are commercially available are hollow point designs to increase their killing power. Look at the difference between the airgun slug penetration and that of pellets recovered from the above penetration tests…

The pellets have changed so little, they look like they could almost be used again (do NOT do that!).

By comparison, the flat-nosed Bob’s Boattail slug has shortened and expanded noticeably. The hollow point version of the same slug came apart, and in doing so released the energy required to do that, creating a lot of damage to the target.

Why are Slugs so Different?

The simple answer is that because they can be! Generally, they are heavier (higher Sectional Density) than most pellets, and their shape (Form Factor) is superior to most pellets, resulting in a better BC.

This means that they can have a Meplat (flat nose) or hollow point and yet still deliver a larger percentage of their energy to the target. A hollow point, in particular, delivers more of that energy to the target with less penetration.

Many HP pellets are not as accurate as their solid counterparts (there are exceptions). This may partly be because their Center of Gravity (CG) is further aft, and the shorter distance between that and the Center of Pressure (CP) gives them less aerodynamic stability.

Slugs are the opposite. The CP is ahead of the CG, and with a HP slug, that moment arm is increased. They get their stability from gyroscopic spin anyways, and most often the hollow point version of a slug ends up with better accuracy than the flat nosed version. This puts the HP slug in a win, win, win situation!

Commercially available airgun slugs are relatively new. You have been able to buy them for a few years from custom swagers and casters, but really only this year have they gone somewhat “mainstream”.

Almost all of the swaged slugs share a common design. They are a “semi-spitzer” shape with the nose cut off to form what would be a flat of about half the slug diameter (a 50% Meplat). Most have varying sizes of HP or even a large hollow cavity, to cause violent expansion.

Some slugs are actually designed to fragment, coming apart in as little as 2 inches of penetration, with the resulting fragments having almost no residual energy, either remaining inside the target, or traveling very slowly when they exit. This would be an excellent feature for shooting at small pests, where you don’t care about destroying the meat, and you don’t want the slug traveling on downrange to cause possible collateral damage.

Now we’ve started with airgun slug penetration, next month I will go into a bit more detail about hollow point slugs. Stay tuned!