Testing airgun pellet impact w/ Mike Morton

Mike Morton takes up clay shooting – with no shotgun in sight – as he examines the damage done by different types of airgun pellet

One thing all airgun hunters agree on is the need to kill a quarry animal quickly and humanely. The taking of an animal’s life, and it doesn’t matter whether it’s a rabbit, a crow or a rat, is not something to be done lightly.

It’s imperative to make a telling shot that results in a fast and painless death. This is sometimes referred to as the one-shot stop.

One thing airgun hunters fail to agree on, however, is the optimal choice of calibre to deliver this result.

So what qualities are hunters looking for in their ammunition, especially when shot through a sub-12 foot pound rifle? And why do hunters seem to fall into those who favour the perceived gain in accuracy from a .177 pellet, and those who believe in the perceived knockdown capabilities of a .22?

In order to better answer those questions, let’s look at what happens when a pellet hits its target. The most effective hunting pellet doesn’t just punch its way into the quarry animal, it creates serious secondary internal damage as it passes through the animal’s brain, heart or lungs.

A larger wound channel increases the likelihood of severing an artery or damaging nerves in addition to the damage done by the actual path of the pellet itself, so advocates of .22 believe that this larger calibre will naturally do more damage than a .177 pellet.

Then there’s the question of the shape and weight of a pellet. For decades, the roundhead or domehead pellet has been seen by some hunters as being suitable only for target shooting, but is that truly the case?

The story so far

Ballistic gel is a testing medium that is designed to replicate muscle tissue so the effects of a projectile can be determined. It was initially developed by the US Army in the 1950s to test full metal jacket bullets against simulated human flesh, but is extremely useful for testing hunting rounds too – and that includes pellets.

Mike made up a batch of homemade ballistic gel for his previous tests to determine the terminal ballistic properties of certain types of pellet. The gel was made to a formula that closely replicates the FBI standard.

He tested various types of .177 and .22 pellet, and in every case the .22 pellet travelled further than its .177 counterpart. This time, he’s trying to find out if the same thing happens when the test is carried out using clay.

Gel or Clay?

At the start of this year, I attempted to solve some of these mysteries by shooting various pellet types of both .177 and .22 calibres into blocks of ballistic gel.

This gel is designed to mimic animal tissue, although it doesn’t take account of bone, feather or fur. Those results were interesting, and not what I’d expected. In every case, the .22 pellets travelled further than their .177 counterparts, and no pellet deformed at any range.

On impact with an animal, a projectile is capable of creating two types of wounds called cavities– these two being either primary or temporary. 

Four pellets were shot from each rifle in exactly the same conditions and at exactly the same range – 30 yards

The primary cavity, which is also known as a permanent cavity, is the wound channel made by the projectile itself, and will be the same size as the initial hole made by that projectile – in our case the calibre of the pellet.

But when a projectile strikes a quarry animal, it will also cause a temporary cavity. As the pellet hits the animal, the soft tissue will initially stretch as it tries to absorb the energy of the shot.

This tissue will keep stretching until it finally gives way, being violently cut and torn as the projectile travels through it before eventually returning to its normal position.

When shot into the ballistic gel, there was minimal evidence of any temporary cavity. Did this mean the pellets weren’t causing this phenomenon, or was the gel simply too elastic to capture the effect of the pellet?

The four types of .177 pellet chosen to be shot in each rifle were as close a match to their .22 counterparts as possible

The solution was to find a material that would go some way to replicating the muscle tissue properties of ballistic gel, while making a permanent record of the damage done by the pellet. And that meant shooting into clay.

Modelling clay doesn’t pretend to be a stand-in for animal tissue or bone, but what it does do is create a three-dimensional snapshot of what a particular pellet is capable of doing when it strikes its target.

The contenders

In an attempt to settle the .177 v .22 for good (was there ever a real chance of doing that?!) I wanted to gather together four pellets in each calibre that were as similar to their opposite numbers as possible.

I selected ones that were readily to hand, and chose three domehead pellets in each calibre of different weights, to see whether mass or velocity was more important, then chose two ballistic-tipped hollowpoint-type pellets that were as similar to each other as possible.

Lightweight pellets were represented by H&N Sniper Light in both calibres, regular weight pellet were represented by Air Arms Diabolo Field, again in both calibres, while the heavyweight pellets I used for this test were JSB Exact Heavy in .177 and JSB Exact Jumbo in .22.

While it would have been better to have used identical pellets in both calibres, the hollowpoint hybrids I used were Predator Polymag Shorts in .177 and RWS Power Piercing in .22. With the pellet selection sorted, it was time to move on to the medium they were going to be shot into.

Finding a claymate

My local hobby shop sells modelling clay and my wife kindly offered to pick some up for me. She brought home two different types, thinking they were the same. 

But this type of clay is available in oven bake or non-firing forms, meaning the oven bake will remain malleable while the non-firing will start to dry out as soon as it’s unwrapped and exposed to the air.

The whole point of firing into the clay is to see what happens inside. The obvious way to do this is to shoot into a block, then cut it in half and see what’s been going on inside, but the consistency of the clay can make this difficult. 

If it’s too soft, the wound channel will deform when the block’s cut open and you won’t get any meaningful results. 

Similarly, if it’s too hard, then it will also be too difficult to cut open. The solution was therefore to use the shot clay as a mould, making a plaster cast, or plug, of the wound channel, and then peel away the surrounding clay to recover it.

For this reason the oven bake type of clay was the best choice, as the non-firing type would dry too hard for it to be pulled away without damaging the plaster cast inside.

Gearing up

I chose two reliable rifles from my armoury that could be shot off a bipod for this test – my Brocock Bantam Sniper HR in .177 and my BSA R-10 SE in .22. Both guns have a similar muzzle energy when shooting standard-weight domeheads in their respective calibres, so this was as close as I could realistically get to guaranteeing the same muzzle energy for each of the different types of pellet.

One other vital piece of equipment that I needed was my homemade target holder/pellet catcher with some orange Target Spots to use as a sighting tool.

I decided there was no need to re-zero each rifle for each pellet, just to shoot at a Target Spot to determine point of impact and then hold off accordingly when shooting into the clay.


One thing I learned when I carried out my earlier ballistic gel test was to maintain the gel at a constant temperature. FBI-grade gel is stored at around five degrees centigrade for it to maintain the correct density to replicate living tissue.

There is no such measure for modelling clay, so the best plan I could come up with was to store the clay at ambient temperature and shoot all eight pellets in quick succession so the density of the clay would be consistent.

I ended up needing to buy two massive slabs of oven bake clay, each of which was sliced into four smaller blocks, giving me the eight individual blocks I needed for this test.

Being malleable, it would have been possible to shoot the clay, squash it back together and then shoot it once more, but again I wanted all eight blocks to be of exactly the same density, so each one was shot ‘fresh’. The face of each block was also cut square so it presented a uniform surface for the pellet to strike and enter.

The shoot

My target-holder was placed 30 yards away from the firing point, with one block of clay resting on top. Once I was happy with my sighting shots and knew where to aim, a single pellet was fired into the block.

The shoot was carried out on a hot day and the clay blocks were tacky to the touch, leaving some residue on my hands. If you want to try this test yourself, then I’d recommend wearing a pair of latex gloves when handling the clay so you don’t get any sticky residue on your gun.

After each block was shot, I made sure to note down exactly which pellet had been shot into it. If you forget this step, all is not lost.

Get plastered

With the shot clay collected and laid out at the range hut, it was time to make the casts of the various wound channels using a simple mixture of plaster of Paris.

It’s important to mix the plaster thoroughly, as lumps can affect the integrity of the cast. It’s also important to make enough plaster so it can be poured in one go, rather than having to top-up a partially filled and cured mould.

Who would have believed that a tiny pellet, shot at a relatively low velocity, could do so much damage?

With all eight blocks filled with liquid plaster, it was time to wait for it to dry. After 90 minutes, I peeled away the clay from the first four moulds, finding it easy to retrieve and clean off a near-perfect cast, along with the spent pellets.

Having cleaned up the first four plaster plugs, I decided to leave the remaining four blocks a couple of days before removing the clay. This was not a good idea, because although the clay was the oven bake type it had started to harden, making it quite difficult to remove.

I had to use a blunt knife to gently cut and scrape away the clay, but although it took far longer than it should have done, I was rewarded with my final four plaster casts. And in most cases, the spent pellet still came away embedded in the plaster.

The results

This test threw up three interesting pieces of information regarding the depth of penetration of each pellet, the diameter of the wound channel created by each pellet, and the deformation (or not) of each pellet.

In terms of penetration, little had changed when comparing the pellets that had been shot into clay with those shot into ballistic gel. With three of the four pellet types, the .22 pellets travelled further into the clay than their .177 rivals.

The results proved there was little to choose between the two calibres when it came to penetration and stopping power

The exception to this was the heavy category, where the .177 JSB Exact Heavy penetrated 162mm into the clay, while the .22 JSB Exact Jumbo went only 151mm – a 7% variation.

When it came to the size of the channels made by the various pellets, all four .177 pellets created a larger diameter channel than the .22 ammo, typefied by the Sniper Light, which created a 27mm channel in .177 compared with just 20mm in .22 – a difference of nearly 30%.

As with the ballistic gel test, none of the pellets showed any significant deformation, apart from the pointed/hollowpoint types in both calibres. 

While the red polymer nosecone of the Predator Polymag Short had come away from the main body, the pellet itself had squashed and expanded: it truly was a case of a massive mushroom.

Despite it being a pointed/hollowpoint hybrid rather than a true hollowpoint, the RWS Power Piercing also opened out on impact. It’s worth pointing out that these two pellets penetrated the shallowest and created the second-widest wound channels within their respective calibres.

Does that make hollowpoint hybrids the ultimate hunting pellets? 

No, not necessarily. 

Ultimately, it’s not what the pellets do when they hit their target that’s of primary importance, as all eight types are more than capable of the humane dispatch of an airgun quarry animal.

What’s far more important is the shooter’s ability to accurately place that pellet on target. So has this settled the .177 v .22 debate? Let us know what you think!

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