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A box of AR-15 rifle lower receivers await serial number stamping and finishing at the Stag Arms Co. in New Britain, Connecticut. 

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In recent years, following reports of mass shootings, media outlets and legislators have tried to define exactly what makes a weapon an assault weapon, and what makes it so deadly.

More often than not, the term “assault weapon,” or even “semi-automatic rifle” more generally, overlaps with the specific rifle design known as the AR-15, an assault rifle developed for the U.S. military in the late 1950s and early 1960s and which has become increasingly popular.

In 2015, for instance, a New York Times project looked at 19 recent mass shootings and showed that semi-automatic rifles were used in eight of those shootings. And in all eight of those instances, the rifle used was an AR-15 variant.

The rifles also correlate to higher casualties. FBI-defined “active shooter” incidents see 71 percent more fatalities when a semi-automatic rifle is used versus other firearms, according to a study published last year in the Journal of the American Medical Association.

But the deadliness of the AR-15, and any other weapon designed around a similar concept, would not come as a surprise to those who engineered it.

The history

The theory of the rifle was an extension of the “intermediate cartridge” concept used in the WWII German Sturmgewehr. The term refers to the cartridge being intermediate in both physical shape and ballistic performance between a pistol cartridge and a full-size rifle cartridge.

With the advent of mobile, armored warfare, Germany and other countries found that turn-of-the-century bolt action rifles were unnecessarily powerful, being built for trench warfare at ranges in excess of 1,000 yards. At the same time, submachine guns, which fire pistol-sized ammunition at a rapid automatic rate, were too limited in range and accuracy.

The Sturmgewehr filled the gap between the two, providing suitable performance out to 300 meters, while producing less recoil than a full-size rifle round, making the gun more controllable when firing rapidly. Reduced weight allowed the solider to carry more ammunition, which was light enough to be fired out of 30-round sheet-metal magazines that were fully detachable and interchangeable.

When the U.S. Army commissioned designer Eugene Stoner of Armalite to design an intermediate cartridge rifle starting in 1957, the concept was improved further.

While the German and Soviet Kalashnikov assault rifle designs had used a shortened cartridge case and shorter, lighter bullet, the projectile was still of the same diameter as full-size rifle rounds. The American military specified that bullet diameter was to be reduced to .22-caliber, or 5.56 millimeters.

The lightweight projectile would reduce recoil, while it’s higher velocity would make up for the lack of mass. Stoner’s AR-15 proved superior to the Army’s existing M-14 rifle, which was largely an update of the M-1 rifle of WWII, and used a .30-caliber cartridge of a more traditional design.

Military reports from the time express skepticism over the performance of Stoner’s 5.56x45 mm cartridge design. But testing reports from Vietnam validated the idea.

In 1961, the yet-to-be-adopted rifle caught the attention of the Department of Defense’s Advanced Research Projects Agency, which was attracted to the “high velocity small caliber principle,” ARPA wrote in its 1962 report. ARPA sought a rifle that could be used to equip South Vietnamese troops, and the intermediate cartridge concept offered a solution to the issue of soldiers’ physical size.

“The problem of selecting the most suitable basic weapon for the Vietnamese soldier is complicated by his small stature and light weight. The average solider stands five feet tall and weighs 90 pounds,” ARPA wrote.

Field reports showed that the AR-15’s light weight and low recoil made it easier for South Vietnamese troops to use. Their American advisers were quite impressed with its killing power.

Excerpts included in the ARPA report were quite explicit.

“At a distance of approximately 15 meters, one Ranger fired an AR-15 full automatic hitting one VC [Viet Cong guerrilla] with three rounds with the first burst,” one report read. “One round in the head — took it completely off. Another in the right arm — took it completely off, too. One round hit him in the right side, causing a hole about five inches in diameter.”

Another run-in detailed five kills with AR-15 rifles — “back wound, which caused the thoracic cavity to explode; stomach wound, which caused the abdominal cavity to explode; buttock wound, which destroyed all tissue of both buttocks; chest wound from right to left, destroyed the thoracic cavity; heel wound, the projectile entered the bottom of the right foot causing the leg to split from the foot to the hip.”

The guerrilla whose buttocks had been blown off lived approximately five minutes, according to the report. The other four were instantaneous kills.

“Two were killed by AR-15 fire,” read another Special Forces report to ARPA. “Range was 50 meters. One man was hit in the head; it looked like it exploded. A second man was hit in the chest; his back was one big hole.”

Theory

While the lethality may have surprised the soldiers, it didn’t surprise the Army, or Stoner.

For most of the history of firearms, limitations in chemistry and metallurgy had put a ceiling on bullet velocity, placing the emphasis instead on the size and weight of the projectile. But that had changed.

The 5.56x45 mm cartridge designed for the AR-15 used a bullet that was only about a third of the weight of traditional .30-caliber military cartridges, and its overall energy output was lower. The inertia of a bigger, slower-moving bullet would often force it through the human body in a straight line. But the extreme velocity of the lighter projectile caused a violent shattering and tumbling effect on impact in soft tissue, causing bones to burst outward and bullet to fragment into a large exit wound.

Even if the round didn’t strike bone, the lightweight projectile was easily upset from its axis, and would “yaw,” or tumble, creating a large cavity inside the body and often exiting at a very different angle than it came in.

The effect has been observed for decades. A New York Times piece speaking with ER doctors after the Parkland shooting described identical injuries in recent mass shooting victims. Just this past June, CBS’ 60 Minutes ran a similar feature complete with lab testing of 5.56x45 mm AR-15 rounds on gelatin meant to simulate human flesh.

In 1967, amidst reliability problems with Colt’s early production runs of the new M-16, Stoner was called to testify before a select panel of the House Armed Services Committee, although by that point he had not been involved with the AR-15 project for some time.

At one point, the chair of the hearing, Rep. Richard Ichord of Missouri, asked Stoner to explain the high-velocity intermediate cartridge concept.

“The reason I asked that question, one Army boy told me that he had shot a Vietcong near the eye with an M-14 and the bullet did not make too large a hole on exit, but he shot a Vietcong under similar circumstances in the same place with an M-16 and his whole head was reduced to pulp,” Ichord said. “This would not appear to make sense. You have greater velocity but the bullet is lighter. The foot-pounds are still going to be less, if it is lighter.”

“There is the advantage that a small or light bullet has over a heavy one when it comes to wound ballistics, even for the same velocity. But, of course, the velocity helps,” Stoner replied.

“What it amounts to is the fact that bullets are stabilized to fly through the air and not through water or a body which is approximately the same density as water,” Stoner continued. “And they are stable as long as they are in the air. When they hit something they immediately go unstable. In other words, your spin rates are determined in air, and not in fluid.”

A .30-caliber M-14 bullet might stay stable through the human body, Stoner said, “while a little bullet, being as it has a low mass, it senses an instability situation faster and reacts much faster. So, therefore, this is what makes a little bullet pay off so much in wound ballistics. As soon as it gets into an unstable portion, it tends to tumble faster, because its mass is lower.”

The 5.56x45 mm cartridge has since become the standard assault weapon round, at least for NATO and other Western-aligned countries. Russia and China also developed their own high-velocity, small-caliber cartridges since the introduction of the AR-15.

While the 5.56x45 mm cartridge was designed for military purposes, it has other uses. The round was adopted commercially as the .223 Remington, and the cartridge is used for target shooting and varmint hunting. Because of this, the National Shooting Sports Foundation suggests the term “assault weapon” be replaced with “modern sporting rifle.”

This history is fundamental to debates over gun reform. The AR-15 and its cartridge were designed together to create a rifle optimized to kill humans, but the rifle and ammunition have since become a mainstay of civilian hobby shooters.

This isn’t a new revelation. Eugene Stoner knew exactly what the gun was supposed to do when he designed it 62 years ago.

Email Zack at zhoopes@cumberlink.com.

This article originally ran on cumberlink.com.

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