Admittedly, I'd rather not be shot with either, but if I had to choose, I'd take a round from the AK47 over the M4 any day of the week. To understand why, it's important to have a very basic look at the physics behind terminal ballistics, in this case being the science of what happens when a penetrating missile enters a human body. The first place to start is the Kinetic Energy Equation:
KE = ½ M (V12 – V22)
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 make sense, and 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 by an AK than 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 dissipates into the target than the mass.
Bang. (U.S. Air Force photo by Staff Sgt. Levi Riendeau)
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, as to dissipate as much energy as possible into them, and hence do maximal damage.
M4 Carbine. (U.S. Air Force photo)
The next concept to grasp is that of permanent cavitation versus temporary cavitation. Permanent cavitation is the hole that gets 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 AK47 round of 7.62mm diameter at its widest point passes cleanly through a target, it will leave a round 7.62mm hole (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, then 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, as they have a tendency to destabilize, and the heavier back-end of the bullet will 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.
(U.S. Marine Corps photo)
Far more damaging than the permanent cavity left by a projectile is the temporary cavity that it creates. Anyone who has ever 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 (below) 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 tract, 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 of a 5.56x45mm round (same as the M4 fires) hitting ballistic gelatine in slow motion. After watching, the medical provider can begin to appreciate the damage that gets done to tissues by the pressure wave of the temporary cavitation.
(Brass Fetcher | YouTube)
Another characteristic of the M4 round is the tendency for the bullet to disintegrate if it strikes tissue at a decent velocity. Despite being a jacketed round, owing to it being smaller, lighter, and faster than an AK47 projectile, it tends to yaw faster once it hits tissue and the shearing forces on the bullet once it is traveling at 90 degrees 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 AK47 round, being slightly heavier and slower than the M4 round will have a tendency to remain intact as it strikes tissue, and whilst it will penetrate deeper, it tends to remain intact and not yaw until it has penetrated much deeper than the M4.
The video below 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 AK47 round without it yawing significantly or disintegrating.
I once saw a good case study illustrating this point nicely where a casualty had sustained an AK47 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 1 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 made a good recovery. That one is a story for another time.
(The Ammo Channel | YouTube)
Although an unpleasant injury to have, the fact that the AK47 round was travelling slower than an M4 at the same range would have been, coupled with the fact that the projectile remained intact and didn't yaw significantly as in 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 AK47 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 AK47 wounds over the years and some relatively minor ones from M4s, so it all depends.
The main take home points for medicos are to be aware of the magnitude of damage that can be caused by the temporary cavitation resulting from high velocity missile wounds, and also if you find an entrance wound, there's no telling where in the body the projectile might have ended up!