Placeholder Image

The baseball practice gunman reportedly fired 7.62 ammunition, two versions of which are shown here. The rows at far left and right are hollow-point. [Flickr user Eric]

Mass Shooting

The Simple Physics That Makes Some Bullets Deadlier Than Others

How higher speed, greater mass, and more surface area increase the damage that rounds can do to human bodies.

The bullet that struck U.S. Representative Steve Scalise last week was travelling at somewhere between 1,100 and 2,600 feet per second. The projectile, a 7.62 x 39 bullet, hit the House majority whip with between 370 and 1,550 foot-pounds of force. The rifle round, which is longer than a pistol projectile, likely also began tumbling after its point collided with his hip. That meant that the tip didn’t just bore straight through him, but rather that the whole length of the projectile rotated over and over through Scalise’s body, ripping a wider hole and distributing a bigger shock wave throughout his bones and tissue.

Scalise, a Republican from Louisiana whose condition has been upgraded to “serious,” suffered broken bones, ruptured blood vessels, and shredded internal organs. The gunman was able to inflict such extensive damage from across a baseball field because he was using a semiautomatic rifle that may have qualified as an assault weapon, akin to a standard infantry combat weapon. Some basic physics explains why the choice of weapon, or more precisely, the choice of bullet, enabled the suspect, James Hodgkinson, who was killed by law enforcement officers, to wreak so much havoc.

Projectile weapons work by transferring kinetic energy to a target, which ripples out as a shockwave through tissue as the bullet plows through the body, leaving a cavity in its wake. The amount of energy a bullet radiates into a target is determined by a simple formula taught in high school: It’s the product of one half the projectile’s mass times the square of the velocity. The energy delivered to the target increases geometrically along with increases in mass, and exponentially with increases in velocity. The larger a projectile’s surface area, the greater its ability to transfer its energy to the target, instead of simply penetrating straight through.


According to the Federal Bureau of Investigation, Hodgkinson was armed with a rifle that shot 7.62 caliber bullets. In a statement, the FBI said he used a modified an SKS , the predecessor to the AK-47 which shoots the same bullets. While typical SKS rifles do not qualify as assault weapons because they have a fixed magazine and stock and lack a pistol grip, Hodgkinson’s gun would have met the criteria. It was modified to accept a detachable magazine and equipped with a folding stock. 

The 7.62 round is not as fast as the .223, the round fired by the AR-15 (America’s most popular assault rifle), but it hits its target with more energy. Tests show that while it travels at 1,154 feet per second at 500 yards, slower than the .223 at that distance, the AK’s bullet can deliver 370 foot-pounds of force, more than the AR. That’s because the 7.62 is almost twice as massive as a .223. In the technical language of ballistics engineers, the standard unit of measurement for a bullet’s mass is grains of metal, and depending on the manufacturer, a .7.62 can weigh in at 125 grains. A .223 is just 55 grains.  

The difference can be seen with pistol rounds, as well. Because it’s difficult to pack enough explosive powder into a cartridge that’s supposed to fit within the shooter’s pistol grip, handgun designers typically try to increase firepower by sticking a bigger piece of metal on the end of a round. The .45, for instance, is slightly slower than the 9mm, but features a more massive projectile, with 250 grains of mass compared to the 9mm’s typical 115 grains.

Since the mid-2000s, gunmakers catering to consumers who increasingly buy weapons for self-defense have produced fewer of the small-caliber handguns like the .25 or .32, and more of those 9mm and larger.

Dr. Peter Rhee is a trauma surgeon who operated on victims of the 2011 shooting at a political event in Tucson, Arizona, in which six people were killed and 13 wounded in a hail of 9mm bullets. Rhee explained that the more massive the projectile, the more severe the wound.

A .22 will kill you, but it won’t blow your head apart,” he said. With bigger bullets, “you will make bigger holes.”


Assault rifles don’t shoot the largest bullets on the market. In fact, the .223 projectile, a common round for the AR-15, is not much larger than many .22 rounds like the Hornet, typically used for youth shooting sports, target shooting, and hunting varmints. The .223 weighs in at 55 grains, while the .22 is usually 45 grains or smaller.

What makes the .223 potentially deadlier than the .22 is its velocity. When the .223 exits the barrel of a gun, it flies at more than 3,200 feet per second, and is still going 1,660 feet per second after traveling 500 yards. The .22, meanwhile, leaves the muzzle at 2,690 feet per second, and slows to 840 feet per second at 500 yards. At that long distance, the .223 will slam into its target with almost twice the speed of the .22. The .223 is carrying 335 foot-pounds of force, while the .22 carries 70 foot-pounds.

Slow-motion videos of ballistics tests clearly illustrate this difference. Watch the .22 and the .223 tested on blocks of ballistics gelatin, a material that mimics human tissue. The .223 generates a far larger shock wave, and penetrates farther, than the .22.

Developed in the early 1960s, the .223 round was first used in Vietnam. The U.S. military collected reports of its effects on the first Viet Cong combatants to be shot with the bullet. Here’s what they found it did to those enemies’ bodies, as republished in the New York Times reporter C.J. Chivers’ history of the assault rifle, The Gun:

Surface area

In the macabre world of terminal ballistics, the study of what happens when projectiles actually strike, much can be made of over-penetration: the phenomenon of bullets passing clean through their targets. That’s a problem, because the force that continues to propel the bullet has essentially been wasted, not transferred to the target.

That problem is solved by making more of a bullet’s surface area actually come in contact with its target. Rifle rounds have an elongated, oblong shape and so they often tumble. When they strike, the bullets rotate, and more of their surface hits the body.

Bullets can also increase the surface area that transfers energy to targets by expanding, as in the case of hollow-point rounds. The wider bullet also creates a bigger cavity in whatever it passes through. “If a projectile has features that would cause it to expand upon impact,” U.S. Army engineers Donald Carlucci and Sidney Jacobson explained in the textbook Ballistics: Theory and Design of Guns and Ammunition, “it will cause greater trauma.”

Hollow points also create a larger temporary cavity. The temporary cavity is the hole in the target that briefly expands after immediate impact, only to collapse a fraction of a second later. While the tissue affected by the temporary cavity isn’t completely destroyed like that in the direct path of the bullet, it does suffer trauma, especially if the bullet travels past less-flexible parts of the body, like bone or dense organs like the liver, which can shatter or rupture from the shockwave.

In one case study of a 24-year-old man struck in the chest by a .40-caliber hollow-point pistol round, doctors found organ damage outside the direct path of the bullet. Though the patient survived and only had a small wound, he suffered extensive contusions throughout his lungs.

Dr. Leana Wen, a trauma surgeon and the health commissioner for Baltimore, wrote a column in the New York Times about the especially devastating effects of expanding rounds: “often pulverizing bones, tearing blood vessels and liquefying organs.” She remembered one young man who was admitted to her emergency room with an abdominal wound from a hollow-point round.

“Blood poured out of his chest cavity,” she wrote. “The bullet had disintegrated his spleen and torn his aorta. Four ribs had essentially turned to dust. The damage was far too extensive. He died in our E.R. He was 15.”

Ballistics engineers try to balance speed, mass, and surface area to make the most effective round possible. At The Trace, we examine what happens after bullets are sold, shot, and wound human beings. The damage is often extensive, and not just for those who die.

Shooting survivors and their caregivers describe a lifetime of chronic pain, repeated surgeries years after being shot, and challenges to everyday living that come from losing parts of vital organs, like a woman who has to buy her son expensive protein shakes after he lost much of his stomach.

We are currently looking for shooting survivors to discuss their experience and their recovery. If you have been hit with any kind of bullet — large, fast, expanding, or otherwise — please fill out our survey about your experience after being shot.

This post has been updated to include information released by the FBI regarding the weapon used in the shooting.