The never-ending caliber debate can be tiresome, but then again occasionally there are good and useful additions to the body of knowledge.  Because of my son being deployed, I have always known and understood (not first hand, but second hand) the utility and superior performance of the 5.56mm in the field (and when I say “field,” I mean Fallujah, not your local shooting range where your know-nothing, tacticool weekend warrior waxes stupid in front of everyone).

I have also heard from his teammates about being shot by the 7.62×39.  Specifically, Corpsman Prince gave me details of his experience.

He was very kind and friendly, well trained, in excellent physical condition, and had absolute commitment to his fellow Marines.  He showed me his wound from Iraq within several days of returning.  A round from an AK-47 had entered through the front part of his lower thigh, ricocheted up his thigh, and exited out of the very upper part of the back of his thigh.  Entry and exit wounds (now scars) were at least a foot apart.

Corpsman Prince stayed in Iraq and did his own rehabilitation during the deployment.  The hardest thing about the experience, he told me, was getting enough pairs of clothing after each successive pair became blood stained.  The more interesting thing about what happened that day with Corpsman Prince was what happened to his fellow Marines.  He wasn’t the only one who was wounded in that engagement.  Several other Marines were also wounded, and Prince had to treat them before he could treat himself.  He did so while bleeding out.

The exit wound was small, virtually as small as the entry wound (if the scar is any indication of the wound size when it happened).  The scars I saw were identical in size.

But the specific addition I wanted to mention has more authority than anything I can muster.  It comes via Defense Review, from Tacmed Australia.  I will quote at length, and then offer up a disagreement and an agreement.

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 (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 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. 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.

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 a 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 destabalise, 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.

Here is my disagreement so far.  I’m not certain that the author understands the distinction between cavitation and creating a cavity (or perhaps it is that he isn’t clearly delineating the difference).  Cavitation has to do with the formation of voids due to change of state of a liquid to a gas, whereas some of what he is discussing has to do with the yawing and fragmentation of the 5.56mm round in tissue.  There are both of these effects at work.

We’ve discussed the rocking back and forth in flight that occurs with the 5.56mm round, based on the very well researched paper Small Caliber Lethality: 5.56 mm Performance In Close Quarters Battle.  This rocking back and forth during flight, which occurs even with boat tail 5.56mm, causes rapid yawing when it enters tissue.  Fragmentation ensues, ensuring multiple fragments take different trajectories through tissue.  This may be what the author is referring to when he mentions permanent cavity.  Or then again, he may be referring to the high velocity of the 5.56mm round, and as I said above, both effects are at work.

But by mentioning Newtonian physics, he is on to something.  It’s not only the energy of the projectile that’s important, but the velocity term is a function of the exit velocity, which means that it’s also important how much energy gets deposited in the object.  If the bullet doesn’t stop, fragment, or otherwise leave its energy in the object, it will not do as much damage as one which does.  It’s the same way with, for example, radiation.  A high energy gamma (e.g., from N-16) can pass through a piece of tissue without ever interacting with orbital electrons and ionizing atoms.  Thus, high energy photons can pass through a body without causing dose, depending upon depth, energy, etc.

In order to demonstrate the point, he provides two videos, one of 5.56mm in ballistic gelatin, and one of 7.62 in ballistic gelatin.  But the most impressive thing the author does is convey his own experience treating wounds from 7.62 mm and 5.56 mm rounds, and then show you what an X-ray looks like of an upper thigh and femur that has been hit with a 5.56 mm round from a couple of hundred meters.

As I said, this was from a couple of hundred meters.  Ponder on that a bit.  While this article doesn’t specifically address the .308/7.62×51, some of the principles are applicable.