Post-Parkland: Yes, the AR-15 is a very deadly military weapon

Introduction

I advise readers in advance that parts of this post will be both technical and probably unsettling. I am going to describe the wound ballistics of the 5.56mm round fired by the civilian-model AR-15 rifle, which is the civilian version of the US military's M4 carbine rifle. The only difference between the two weapons is that the M4 has a selector switch that enables it to fire fully automatic. That is, if the shooter depresses and holds the trigger, the weapon will will continue to fire until its ammunition supply in exhausted (see end note).

Both weapons will fire semi-auto, where a trigger pull shoots one round only. Trained shooters can fire very rapidly that way by pulling and releasing the trigger, but the rate of fire will not be nearly as high as full auto. Nonetheless, it can still be very high.

In ballistics, the general principles of which I learned at the US Army Field Artillery School, there are three phases for rifle (or artillery) ammunition: barrel ballistics, flight ballistics, and terminal ballistics. For rifles, terminal ballistics are usually called wound ballistics - what happens to the bullet when it strikes a target. For this discussion, the target is a human body.

I gained my expertise, such as it may be, in this arcane subject area during the years I served as a principal staff officer of US Army Criminal Investigation Command, which is the Army's version of NCIS (for which there is an extremely unrealistic series on TV). Investigators become specialized over time in diverse forensic fields ranging from accounting to blood-pattern analysis to wound ballistics. They were my tutors. So that is where I learned most of these factors. I asked CID agents to review my final draft of this article and received very learned and concise additions and emendations, which I have incorporated. One retired agent who reviewed it spent 40 years in forensic ballistics work, including wound ballistics. He had investigated a large number of cases involving military rifles. I am grateful for the agents' contributions.

This is, btw, a long post of several minutes reading time.

History of the rifle and ammunition

After World War II the US Army and NATO countries adopted 7.62mm rifle rounds as the standard. I am, not trying to sound pedantic, but that means that the bullet is 7.62 millimeters in diameter. "Caliber" is expressed as a decimal of inches, for example, .30-caliber means the bullet is 3/10 inch in diameter.

By the latter 1950s, the U.S. began working to find a different rifle round for the NATO standard. Finally, the Armalite Rifle (hence, "AR") company produced a rifle that in 1963 was adopted by the US Army as the M16. Armalite worked on the rifle in concert with Remington Arms for the ammunition, which was type-classified also in 1963 as the 5.56mm M193 round. However, while the 5.56mm round was adopted as the NATO standard in 1977, it was not the US the M193 that was adopted because several NATO nations considered the wounds the M193 produced to be so devastating that they approached inhumane.

The M16 rifle itself had serious teething problems in the Vietnam war. Early models were prone to misfeeding and jamming. These were finally corrected and the rifle and ammunition became extremely lethal tools in the hands of American soldiers and Marines. Because the M16 was lightweight and the M193 round produced low recoil compared to previous military rounds, US troops were able to achieve very high rates of accurate fire, much higher on both counts than the 7.62mm round or its .30-06 WW2 predecessor. (.30-06 means that it is a .30-caliber round that was adopted by the military in 1906.)

The NATO standard 7.62mm round, left, and the 5.56mm round next to a AA battery. 

Why is the 5.56mm round so deadly in school shootings?

The 5.56 round is so devastating is because of its ballistic characteristics and its very high velocity. Barrel ballistics are not significant for this discussion.

Flight ballistics: The bullet exits the muzzle of the AR-15 (or M4) unstable. The bullet is not merely spinning around its long axis (the front to rear line). It also "yaws" circularly, up to 4 degrees off center line (a form of gyroscopic precession). At about 100 meters, however, the yaw disappears and the round flies very stable out to about 400 meters, when it starts yawing again.

Victims of school shootings are all shot at ranges of much less than 100 meters. So the bullet strikes them while still yawing. That directly affects what happens to them,

Terminal, or wound, ballistics: There is a term or art among law-enforcement officers called, "instant incapacitation." It means a firearm wound that is so severe that the stuck person becomes functionally incapable either immediately or within very few seconds. Instant incapacitation is caused by two things: First is massive and sudden loss of blood. Second is severe interruption of the central nervous system. The 5.56 does both.

1. The 5.56 round exits the rifle at just more than 1,000 meters per second, about three times the speed of sound.  When the 5.56 bullet hits a human body, it immediately begins to decelerate. This bullet's length to width ratio is high. The nose of the bullet begins to decelerate but the rear is still traveling supersonic. The rear is going faster than the nose. This causes the rear to overtake the nose, meaning that the bullet begins to tumble end over end. This tumbling in turn causes the bullet to fragment not quite completely and the fragments travel through flesh, bounce off bone into new directions and sever nervous-system connections. This tumbling is enhanced if the bullet is yawing at impact, as every bullet fired during a school shooting is. That is part one of what causes instant incapacitation.

2. Many ballisticians also say that the supersonic shock wave, shaped like a cone pointed in the direction the bullet was flying, enters the entrance wound and expands it rapidly for a distance into the body (how much depends on the location of the entrance wound and the angle). This causes excess bleeding over what the bullet would cause alone. This effect is called "hydrostatic shock," but not all armaments ballisticians agree that it is significantly damaging. In fact, while sound travels four times faster through human tissue than through the air, sonic waves have been proven not to damage the body.

Cavitation: Any high-velocity bullet (and almost all non-HV bullets, too) not only creates a cavity in the body corresponding to the width of the bullet, they also create large-diameter cavities from the sudden displacement of tissue caused by the speed of the bullet. This is called "cavitation" and highly variable from one weapon to another. A radiologist who helped treat victims at Parkland described it this way:

The bullet from an AR-15 passes through the body like a cigarette boat traveling at maximum speed through a tiny canal. The tissue next to the bullet is elastic—moving away from the bullet like waves of water displaced by the boat—and then returns and settles back. This process is called cavitation; it leaves the displaced tissue damaged or killed. The high-velocity bullet causes a swath of tissue damage that extends several inches from its path. It does not have to actually hit an artery to damage it and cause catastrophic bleeding. Exit wounds can be the size of an orange.

However, the effects of cavitation depend on the tissue affected. Muscle tissue is much less affected and, given survival of the victim, muscle tissue will recover. Vital organs are not so hardy. They can be badly damaged by cavitation but even so, actual destruction is caused by bullet fragmentation much more than cavitation.

Here are photos of cavitation in ballistic gelatin hit by a 5.56 round. The top photo is of a non-yawing round, the bottom of a yawing round. At the far left of each photo is the bullet angle of attack.

 As you can see, while both impacts are horrific, the yaw "wound" is truly devastating. Earlier in the same article, the radioligist observed,

I was looking at a CT scan of one of the mass-shooting victims from Marjory Stoneman Douglas High School, who had been brought to the trauma center during my call shift. The organ looked like an overripe melon smashed by a sledgehammer, and was bleeding extensively. How could a gunshot wound have caused this much damage?

The reaction in the emergency room was the same. One of the trauma surgeons opened a young victim in the operating room, and found only shreds of the organ that had been hit by a bullet... . Nothing was left to repair—and utterly, devastatingly, nothing could be done to fix the problem. The injury was fatal.

These facts are why I reject as unfounded - indeed, invalid and misleading - that the 5.56mm round is nothing special compared to other rifle rounds and is not very powerful at all, a claim that was explicitly made in  Tennessean op-ed only yesterday (Feb. 25), "Why blaming the AR-15 for mass shootings is misguided." The author, Robin Patty, "is a disabled veteran and a former Special Forces operator who resides in Murfreesboro." While I thank her for her service, neither being a disabled veteran (so am I) or a Special Forces operator bestows special expertise in this area.

In her op-ed, Ms. Patty writes of the AR-15 firing a 5.56mm round,

It’s not powerful, so much so that some states don’t allow the cartridge that it fires to be used on deer.  

This weapons system was never designed or intended to be used to hunt any kind of game animal. It was developed by the Defense Department to do one thing only: kill humans beings of enemy armies as quickly as possible. For that purpose, it is extremely powerful. That some states do not allow it to be used to hunt deer is true. It is also irrelevant.

It’s not military grade. It simply looks like a military rifle, as the M16/M4 are all capable of automatic fire and the AR-15 is not.  

As I wrote above, that is the only difference between the military M4 and the civilian AR-15. I again say it is true but also irrelevant. Perhaps (as in maybe) Cruz could have mowed down 45 killed and 25 wounded if he had been shooting an M4 on full auto. So what? Does that make 17 dead and 12 wounded somehow less serious or less urgent for actions to prevent another repeat? Of course not.

When discussing school shootings - and not other kinds of murders or even other mass shootings - there is a unique threat in the 5.56 round fired from the AR-15 rifle.

That is why I find it difficult to oppose raising the legal age to 21 to possess these weapons because frankly, an average 18-year-old today is mentally and morally at about the level of a 14-year-old (and often younger) of any prior generation. At the same time, though, Cruz is the only school mass killer under age 21; all the others were mid-twenties except Lanza, 20, and he murdered his mother to get his hands on her AR-15 that he used to shoot school children.

So while I will not oppose raising the legal-possession age, I also do not really think it will much matter. But I will go one step further: if 18-year-olds are too immature to own a rifle, then we sure as H E double hockey sticks should not let them vote, either.

Coming soon: The other op-ed in Sunday's Tennessean saying, "A solution to ending mass shootings: Ending sales of guns to civilians," and why it is even more gravely in error than Ms. Patty's op-ed. In fact, it is downright looney. But that is for later.

Related: Mass shootings: "Hope is not a method and wishes are not plans"

End note: M4 carbines issued to soldiers for combat will fire full auto, but only three rounds at a time. Extensive testing by the Army showed that because of recoil, the soldier's aim is degraded after the fourth round so that it and subsequent rounds miss. Therefore, those weapons are modified so the soldier will shoot three, stop, re-aim, shoot three, stop, etc.

Also, a commonly-used round in AR-style guns is the .223-caliber round rather than the 5.56mm. They are very similar but are not identical. However, the terminal ballistics, which are what matters, of the .223 are the same as for the 5.56.