Doctors: High-velocity Orlando rifle inflicts 'devastating' wounds

With their diameter roughly one-third that of a dime, the bullets apparently used in the Orlando mass shooting were small.

But because they travel at nearly three times the speed of sound, they deliver a punch well beyond the hole made by the bullet, pulverizing nearby blood vessels and turning soft tissue to jelly.

Combat surgeons who have treated wounds inflicted by this type of high-velocity rifle, the AR-15 class, say the weapons scare them.

"The wounds are just otherworldly," said Penn Medicine trauma surgeon Jeremy W. Cannon, an expert marksman who served with the Air Force in Iraq and Afghanistan. "You're talking big, giant cavities and a hole you can put your fist through."

John M. Porter, chief of trauma at Cooper University Health Care in Camden and a former Army trauma surgeon, said injuries caused by high-velocity rifle rounds are "much harder to fix" than those from a handgun.

"It actually puts kinetic energy into tissue that it didn't hit," Porter said. "It can go next to the blood vessels and still destroy the blood vessels. It can go next to the liver and still destroy the liver."

Those engaged in the gun-control debate have argued over whether the labels "assault" and "military-style" should be applied to such weapons, which are made to fire just one round with each squeeze of the trigger. And round for round, there are some hunting rifles that inflict more damage than the model authorities have said was used in Orlando, a Sig Sauer MCX.

But trauma surgeons say they are concerned by the MCX and others in the AR-15 class because they feature both a high muzzle velocity and the ability to reload automatically from a magazine with 30 or more rounds.

The shooting early Sunday at a nightclub in Orlando left 49 victims dead and dozens wounded, some with mangled bones and serious damage to internal organs. CNN reported that one patient whose pelvis was shattered needed 90 units of blood and four surgeries to stop the bleeding.

The damage inflicted by a firearm depends on many variables, such as the firing distance and whether the bullet passes through the target. But ultimately it depends on a straightforward formula, said Alexey Aprelev, an assistant professor of physics at Drexel University.

The key is kinetic energy: one-half the mass of the bullet times the square of its velocity.

Translation: the bigger and faster the bullet, the more energy delivered to its target. And because the velocity is squared, it is especially important.

As a team of University of Arizona physicians wrote in a recent issue of the Journal of Trauma and Acute Care Surgery:

"If the mass is doubled, then the energy is doubled. However, the velocity of the bullet is a more important determinant of tissue injury, because if the velocity of the bullet is doubled, the energy increases four times."

The muzzle energy from an AR-15-style rifle firing a .223 caliber round is about 1,300 foot-pounds - more than triple the muzzle energy from a 9 millimeter handgun firing a standard bullet that weighs 115 grains (about a quarter of an ounce), the Arizona team wrote.

Though damaging, rifles account for a small proportion of urban homicides. In Philadelphia from 2010 to 2014, rifles or shotguns were used in 2 percent of homicides for which a weapon was identified - 17 out of 847, according to police.

The breadth of damage caused by a high-velocity rifle round depends on what type of tissue it hits, said emergency surgeon Brian P. Smith, an assistant professor at the University of Pennsylvania's Perelman School of Medicine.

And the true extent of damage may not be readily apparent when surgeons first go in to stabilize the patient, especially if it strikes soft tissue. That is because the energy dissipated from a high-velocity bullet can damage blood vessels over an area far wider than the path of the bullet itself, Smith said.

That disruption eventually kills the surrounding tissue because it is not receiving adequate oxygen and nutrients, he said.

The dead tissue must be removed, and surgeons may try to rearrange surrounding tissue to patch up the damaged area.

If a high-velocity round strikes bone, it wreaks a different kind of havoc, according to Penn's Cannon, who said he had qualified as an expert with the M16 - the military cousin of the type of weapon used in Orlando.

"The fractures are horribly splintered," Cannon said. "The bone is just in multiple small fragments."

When stationed overseas, Cannon saw wounds inflicted by the military versions of these rifles, which can deliver three bullets in one burst. But he said the impact of the individual bullets was the same as what was seen in Orlando: deadly.

tavril@phillynews.com215-854-2430@TomAvril1