Lives May Have Been Saved If Holmes or Lanza Had Automatic Weapons

In the Aurora, CO,  theater shooting, James Holmes armed himself with a pump shotgun, a semiautomatic rifle and a handgun.

It's reported that his semiautomatic rifle jammed after 30 rounds. I don't know how many of those rounds killed people, but if he'd been shooting a fully automatic or burst-mode weapon the count would almost assuredly have been lower.

Automatic weapons recoil and the barrels climb with each round. Gunners with fully automatic weapons are trained to fire in 3-5 round bursts to compensate for the increasing loss of control. If Holmes was using an M4 with a burst mode, he would have been firing three rounds with each pull of the trigger. Odds are that 1 or 2 rounds from each trigger pull would have either hit the same target or completely missed. No matter what, he would have been reduced to ten targets, ten trigger pulls, from those thirty rounds, not 30 targets. If he'd been using a model with only a full-auto mode, he almost certainly would have had even fewer aimed targets and worse accuracy -- limiting yourself to a three-round burst isn't easy at 12 rounds/sec.

An automatic weapon causes a shooter to burn through ammunition faster, but without a corresponding increase in lethality against unarmed targets.

He used a semi-automatic handgun, a Glock 22. If he'd had the fully automatic Glock 18, he almost surely would have been less effective. Machine pistols are very hard to control. If he'd used one in automatic mode, most of his rounds would have been wasted.

Automatic fire is a tool of warfare. Its purpose isn't to make killing a specific target easier, it's to reduce the ability of an armed attacker to fire back at you and your allies. The term is "suppressive fire" -- as long as there are lots of rounds heading in the general direction of the enemy, they have great incentive to keep their heads down, which means they aren't shooting back. This allows your allies to move, to aim, to bring the battle to the enemy.

For anything other than suppressive fire, they're arguably a huge waste of ammunition.

So, equipping a gun with a feature that causes rounds to be fired less accurately and less efficiently would seem to be a way to save lives in a mass shooting situation. Every round that misses its target is one less round the attacker has to use against defenseless victims. You don't need suppressive fire when no one can shoot back.

I don't know if it would have saved any lives in the case of Adam Lanza and the Sandy Hook shootings, but it certainly wouldn't have cost any. He was shooting metaphorical fish in a barrel. Unarmed women and children. He had twenty minutes all to himself. (Update: It's since been said that Lanza was active for only five minutes before he killed himself. The same logic still applies.) He didn't have to hurry. He didn't have to fire quickly or change magazines under fire. He could have been using a lever-action Marlin .30-30 with similar effect.

However, there is a possibility that his possession of a fully-automatic weapon would have had a positive effect. An M16 can fire more than 12 rounds per second. An undisciplined shooter can empty a 30-round magazine in full-auto mode in a matter of less than three seconds. This causes the barrel and action to heat up, increasing the chance of malfunctions, and it wastes enormous amounts of ammunition. Lanza was just an angry kid, not a trained soldier. It's entirely plausible that with a fully-automatic weapon in his hands, he wouldn't have been able to resist the temptation to spray fire in full-auto mode. He could have depleted his entire stock of ammunition in mere minutes, saving many lives, as most of the rounds would have been wasted.