Automatic probably would have had fewer since it would have missed with most the shots. Lethality like that requires controlled and aimed fire with individual shots.

As much as USA is pro-gun, I thought automatics are not sold?

Regardless as the other comment says, firing on full auto is a waste of ammo. Back in service our first range practice we were given the Rambo-fantasy; hip firing at full auto at 10 metres out. No one hit any of the targets. It was a lesson to us that firearms, as easy as they are to use, needed at least some skill to be effective.

Highly unlikely unless he built it himself. Machine guns (automatic weapons) are extremely expensive (anywhere from the cost of a new car to a new house, depending on the model) and require you to submit to a colonoscopy, administered courtesy of the ATF. The result is that it's extremely rare for a legally-owned machine gun to be used in a shooting (afaik it's only happened once or twice since the ATF introduced tax stamps for them).

The thing that confuses me is why it is the government is able to restrict automatic weapons and weapons above a certain caliber through what is effectively a license system, but isn't able to restrict anything else because it'd run afoul of the 2nd amendment.

Guns should be like cars. No one should feel like they need one (except if it's part of your work, in which case your work should be providing one), and like a car, different kinds of guns should have different licenses. You want a double-barrel shotgun? Okay. You have to go through a week of training to get a basic redneck license and show you have a gun safe (not a cheapo lockbox) to keep it in when not in use. You want a machine gun? Cool. You have to get the super-ultra-deluxe gun owner's license that requires a year of training, authorization from the ATF and FBI, and proof that you have a gun safe to store it in.

Oh yeah, and if your gun is stolen and it was improperly secured and/or you fail to report it in a reasonable amount of time then your license(s) are permanently revoked and you're considered to be an accomplice to whatever crimes were committed with it.

I'd be willing that the last bit would dramatically cut down school shootings specifically.