A bullet with more energy can do more damage. Its total kinetic energy is equal to the one-half the mass of the bullet times its velocity squared. The bullet from a handgun is—as absurd as it may sound—slow compared to that from an AR-15. It can be stopped by the thick bone of the upper leg. It might pass through the body, only to become lodged in skin, which is surprisingly elastic.
The bullet from an AR-15 does an entirely different kind of violence to the human body. It’s relatively small, but it leaves the muzzle at three times the speed of a handgun bullet. It has so much energy that it can disintegrate three inches of leg bone. “It would just turn it to dust,” says Donald Jenkins, a trauma surgeon at University of Texas Health Science Center at San Antonio. If it hits the liver, “the liver looks like a jello mold that’s been dropped on the floor.” And the exit wound can be a nasty, jagged hole the size of an orange.
I will grant that the gun terminology is all over the place (and I was sloppy with mine). It really doesn't matter if the weapon is technically a "rifle" or a "pistol". What does matter is the "semiautomatic" part coupled with high-capacity magazines.
That said, it is still the AR-15 that is the gun of choice for these mass murderers, yet no one can give me any redeeming feature about this specific gun