Admittedly, I’d rather not be shot with either, but if I had to choose, I’d take a round from the AK-47 over the M4 any day of the week. To add a caveat to that statement, I’m talking from relatively close range here—say up to 150-200 meters.

To understand why, it’s important to first take a very basic look at the physics behind terminal ballistics. In this case, consider the science of what happens when a penetrating missile enters a human body. The first place to start is the following kinetic-energy equation:

KE = ½ M (V1-V2)2

Breaking this equation down into its components, we have kinetic energy (KE) influenced by the mass (M) of the penetrating missile, as well as the velocity (V) of the missile. This makes sense; it is logical that a heavier, faster missile is going to do more damage than a lighter, slower missile. What is important to understand is the relative influence that mass and velocity have on kinetic energy, as this is key to understanding why I’d rather be shot with an AK than with an M4. You’ll notice that the mass component of the KE equation is halved, whereas the velocity component is squared. For this reason, it is the velocity of the projectile that has far more bearing on the energy that it delivers into the target than the mass.

The V1-V2 component of the equation takes into consideration that the projectile might actually pass straight through the target, rather than coming to rest in the target. In this instance, the change in the velocity of the projectile as it passes through the target (V1 being its velocity as it enters, and V2 being velocity on exit) is the factor that is considered when calculating how much energy the missile delivered into the target. Naturally, if the projectile comes to rest in the target (ie: no exit wound), then V2 equals zero and the projectile’s velocity as it entered (V1) is used to calculate the KE.

That’s enough physics for now, but you get the concept that the optimum projectile to shoot someone with is one that has a decent mass; is very, very fast; and is guaranteed to come to rest in your target so as to dissipate as much energy as possible into them, and hence do maximal damage.

The next concept to grasp is that of permanent cavitation versus temporary cavitation. Permanent cavitation is the hole left in a target from a projectile punching through it. You can think of it simply like a sharp stick being pushed through a target and leaving a hole the diameter of the stick. The permanent cavity left by a bullet is proportionate to the surface area of the bullet as it passes through the tissue. For instance, if an AK-47 round of 7.62mm diameter at its widest point passes cleanly through a target, it will leave a round, 7.62mm permanent cavity.

If this hole goes through a vital structure in the body, then the wound can be fatal. However, if the bullet passes through soft tissues only, the permanent cavity can be relatively benign. This is a slight oversimplification of the concept, as bullets will rarely remain dead straight as they pass through human bodies; they have a tendency to destabilize, and for the heavier back end of the bullet to want to overtake the front. This concept, known as yaw, increases the frontal surface area of the bullet as it passes through tissue, and hence creates a larger permanent cavity.

Far more damaging than the permanent cavity left by a projectile is the temporary cavity that it creates. Anyone who has watched the TV show MythBusters will have some familiarity with this concept, and it is best demonstrated using slow-motion video imagery of bullets being shot into special jelly known as ballistic gelatin, which is calibrated to be the same density as human soft tissues. What can be seen in these video images is the pulsating dissipation of energy that emanates out from a bullet as it passes through the gelatin. This is a visual illustration of the concept of temporary cavitation, and it allows the viewer to begin to appreciate the devastating effect that a high-velocity missile can have once it enters a human body.

The temporary cavitation is the transfer of kinetic energy from the projectile into the tissues of the target, and as we learned above, is relative to the mass and, more importantly, the velocity of the projectile. As the energy of the projectile is dissipated into the tissues of the target, the temporary cavitation pulverizes structures adjacent to the bullet’s path, including blood vessels, nerves, muscles, and any solid organs that may be in close proximity. For that reason, the high-velocity projectile does not need to pass directly through a structure in the body to destroy it. The higher the kinetic energy of the projectile, the further out from the permanent cavity the temporary cavity extends.

Below is a slow-motion video by Brass Fetcher of a 5.56x45mm round (same as what the M4 fires) hitting ballistic gelatin in slow motion. After watching, the medical provider can begin to appreciate the damage done to tissues by the pressure wave of the temporary cavitation.

Having had the chance to treat dozens of high-velocity missile wounds over my years in the military, I’ve seen firsthand the effect that various rifle calibers can have at various distances, hitting various body parts. Naturally, there is a multitude of variables that come into play when someone gets shot, and no two gunshot wounds are ever going to be the same. The purpose of this article is not to draw any academic conclusions about the ballistics of the AK-47 versus the M4, or argue the merits of one ammo over another, it is to introduce the concepts of the different wounding profiles of permanent and temporary cavities using a couple of case studies.

Below are two examples I was involved with that illustrate somewhat of a comparative study of an AK-47 round and an M4 round striking approximately the same anatomical location, and from roughly the same range (in these cases, 150-200 meters).

This series of photos you can see a particularly nasty M4 gunshot wound, with a small entrance wound in the right lower buttock, and a massive exit wound in the right lateral thigh. The X-ray in the last image shows that the projectile struck the upper femur and demolished the bone, sending secondary bone fragments flying through the tissues and accounting for the majority of the exit wound. The damage done by the pressure wave of the temporary cavity can be appreciated in the first image, with deep bruising extending up the buttock and into the casualty’s lower back. This bruising resulted from the energy dissipated through the tissues pulverizing small blood vessels in its path (think back to the ballistic gelatin video to imagine what went on in the tissues).

The granular material in the middle of the thigh wound seen on the X-ray is an older-generation QuikClot advanced clotting sponge (ACS), which was inserted at the point of injury for hemorrhage control to excellent effect. The bright white fragments on the X-ray are small pieces of the bullet, which had disintegrated on impact with the tissue and bone. This is another characteristic of the M4 round that makes it all the more unappealing to be shot with—the tendency for the bullet to disintegrate if it strikes tissue at a decent velocity.

Despite being a jacketed round, because it’s smaller, lighter, and faster than an AK-47 projectile, the 5.56mm tends to yaw faster once it hits tissue. The shearing forces on the bullet once it is traveling at 90° through the tissue often tears the bullet into pieces, thus creating multiple smaller projectiles and increasing the chances of all of the bullet parts remaining in the target, and hence dissipating more energy. The AK-47 round, being slightly heavier and slower than the M4 round, has a tendency to remain intact as it strikes tissue, and although it will penetrate deeper, it tends to remain intact and not yaw until it has penetrated much deeper than the M4.

Here’s a video from The Ammo Channel of the AK-47’s 7.62x39mm projectile being fired into ballistic gelatin for comparison to the video above of the 5.56x45mm (M4) round. Although the video shows a soft-point round being used, which theoretically should be more destructive than its full metal jacket counterpart, the video still illustrates nicely the significant penetration of the AK-47 round without it yawing significantly or disintegrating.

I once saw a good case study illustrating this point, where a casualty had sustained an AK-47 gunshot wound to the right lateral thigh and we recovered the intact bullet from the inside of his left upper abdominal wall. It had passed through approximately one metre of his tissues and shredded his small bowel, but the projectile hadn’t fragmented at all, and the temporary cavitation hadn’t done enough damage to be lethal. The casualty required a laparotomy to remove multiple sections of small intestine, but he made a good recovery. That one is a story for another time.

The next photo is of a good friend of mine who was shot by an AK-47 from approximately 200 meters while standing right next to me! Fortunately, the bullet passed cleanly through, and after a surgical clean-out the afternoon of the injury, he turned up ready for work the very next day. They breed them tough where he is from!

This image was taken a few days after the injury and the bruising from the temporary cavity of the projectile can be seen along the path of the bullet. The entrance wound is at the top of the left buttock, with the exit being down on the left upper thigh. Although an unpleasant injury to have, the fact that the AK-47 round was traveling slower than an M4’s round at the same range would have been, coupled with the fact that the projectile remained intact and didn’t yaw significantly as it passed through him, meant the wound was nowhere near as devastating as the above-mentioned M4 injury in the same area. It must be noted, however, that the comparison is far from perfect given that the M4 injury involved the bone, with the one immediately above passing solely through soft tissues.

So there it is. All things being equal, when all is said and done, I’d rather be shot with an AK-47 than an M4 on any day of the week. Naturally, as medical responders, it is always important to treat the wound and not the rifle that inflicted it, and I have certainly seen some horrendous AK-47 wounds over the years and some relatively minor ones from M4s. It all depends. The main take-home points for first responders and medicos are: Be aware of the magnitude of damage that can be caused by the temporary cavitation resulting from high-velocity missile wounds, and if you find an entrance wound, there’s no telling where in the body the projectile might have ended up!

This article was originally written by Dr. Dan Pronk. Dr Pronk completed his medical schooling on an Australian Army scholarship and served the majority of his military career with Special Operations Units, including four tours of Afghanistan and over 100 combat missions.