So, this is where your favorite gun-savvy lefty explains some things.

Lately, we’ve heard a LOT about the AR-15, which is the civilian and semi-automatic version of the M-4 used by our military. The rifle is ubiquitous today; until recently, I’d bet 90% of America was within 20 miles of a store that would sell them one this afternoon. They’re not that expensive, and they’re absolutely terrifying — it’s not for nothing that they’re the weapon of choice for mass shootings.

The the AR-15 is really just one of a class of magazine-fed, semi-automatic rifles chambered for a particular bullet type (the proper term is “round” or “caliber”) usually abbreviated to “5.56”. The proper name is 5.56 x 45mm NATO.

It’s not an especially beefy or powerful round in the realm of rifle rounds; most folks hunt deer with far larger calibers. It’s relatively small size, though, makes it very, very well suited for rapid fire because it produces so little recoil (“kick”). Sure, you almost never see a fully-automatic AR used for crimes, but it hardly matters because a semi-automatic version will fire as quickly as you can move your finger.

That small round packs a tremendous punch — especially since it’s usually shot from a platform that allows or even encourages the shooter to keep firing.

So in this context, take a look at this piece by a Parkland area radiologist, speaking about the wounds from the MSD shooting:

What I Saw Treating the Victims From Parkland Should Change the Debate on Guns.

This is from a radiologist with plenty of exposure to handgun wounds. They tend to be relatively simple and manageable, and if the bullet manages to avoid something critical like the aorta or the heart, the patients tend to survive:

In a typical handgun injury, which I diagnose almost daily, a bullet leaves a laceration through an organ such as the liver. To a radiologist, it appears as a linear, thin, gray bullet track through the organ. There may be bleeding and some bullet fragments.

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?

More:

Routine handgun injuries leave entry and exit wounds and linear tracks through the victim’s body that are roughly the size of the bullet. If the bullet does not directly hit something crucial like the heart or the aorta, and the victim does not bleed to death before being transported to our care at the trauma center, chances are that we can save him. The bullets fired by an AR-15 are different: They travel at a higher velocity and are far more lethal than routine bullets fired from a handgun. The damage they cause is a function of the energy they impart as they pass through the body. A typical AR-15 bullet leaves the barrel traveling almost three times faster than—and imparting more than three times the energy of—a typical 9mm bullet from a handgun. An AR-15 rifle outfitted with a magazine with 50 rounds allows many more lethal bullets to be delivered quickly without reloading.

I have seen a handful of AR-15 injuries in my career. Years ago I saw one from a man shot in the back by a swat team. The injury along the path of the bullet from an AR-15 is vastly different from a low-velocity handgun injury. 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.

Let’s quantify this. Wikipedia can help; let’s compare the 5.56 to the most popular handgun round, 9mm.

Firearm ballistics are a complicated area that people LOVE to argue about, but the gist of the system boils down to the bullet’s mass and the amount of energy pushing it forward. The bullets are measured in grams (or sometimes another unit called grains); we talk about energy in terms of muzzle velocity and downrange energy. The difference, as the author notes, isn’t small:

• A 5.56mm round involves a fairly tiny bullet (~ 3-4 grams, so more than a penny and less than a nickel) moving at about 900 m/s, and will deliver on the order of 1,800 joules downrange.
• The 9mm pistol users a heavier bullet (7 to 8 grams, so twice as massive as the 5.56), but it’s moving far slower: usually the neighborhood of 350 or so m/s, so the energy delivered when it his something is also far lower (~ 500 joules).

An AR-15 is also engineered to shoot quickly, and shield the user from almost all the recoil. I’ve shot one several times; it’s very easy to shoot, and very easy to shoot quickly without losing the target. Frankly, it’s easier to stay on target with an AR than it is with most 9mm pistols.

This is why the wounds the Parkland physician saw were so much worse, and why mass shootings end with so many dead: because it’s easy to get a weapon that will fire very many of these very lethal rounds very quickly. And the NRA likes it this way.

Oh, one more thing: Gun violence is clearly a public health problem in the United States, but we don’t study it. Why?

The Centers for Disease Control and Prevention is the appropriate agency to review the potential impact of banning AR-15-style rifles and high-capacity magazines on the incidence of mass shootings. The agency was effectively barred from studying gun violence as a public-health issue in 1996, by a statutory provision known as the Dickey Amendment.

Why is it banned? Because the NRA doesn’t want it studied. Think on that.