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.