It's hard to find definitive statistics on that. If you go looking around you'll see a few cases that pop up. In the early 2000s a police officer (and member of the SWAT team) used his issued MP5 to murder a few people, and sometime in the 70s a police officer found his wife in bed with another man and killed one (or both? I can't remember) of them.
The point is, like you said, it's exceedingly rare.
You're right, the stats on that are somewhat hard to find, or hard to qualify as definitive. I've seen articles that say "NO legally owned fully automatic weapon has ever been used to commit a crime" and I've seen a few articles that say the number is < 10. I don't think I've ever seen anything that even tries to argue that the number is higher than that however.
Guncite only comes up with two cases, one of which may one of the same ones you just mentioned:
There is no real definition of what an "assault weapon" is, and few restrict that to only automatic weapon.
I would say that an AR-15 is most certainly an assault weapon. It is a high power weapon (ignore the rather ignorant people who fail to understand the difference between a .22 and a .223) designed and built for military purposes. Being automatic fire or not has remarkable little relevance to its deadliness.
There is no real definition of what an "assault weapon" is, and few restrict that to only automatic weapon.
Right, because "assault weapon" is a made up term, created by the people at the Brady Campaign and other radical anti-gun groups, to promote their fear-mongering approach to advocating for more gun control. An "assault rifle" OTOH, does have a technical definition, and it does involve a full-auto or select-fire capability. A civilian AR-15 is not an assault rifle. Calling it an "assault weapon" makes as much sense as calling it a "gandering gillifrous".
It is a high power weapon (ignore the rather ignorant people who fail to understand the difference between a .22 and a .223)
Meh. A .223 is still a relatively low-powered round in the grand scheme of things. A typical .223 round has less kinetic energy than a standard 30-06 round which is used for hunting all over the United States. In fact, .223 is illegal for hunting deer and other large game in some states, because it isn't lethal enough.
Calling it an "assault weapon" makes as much sense as calling it a "gandering gillifrous".
It's purely coincidental that it is a weapon enamoured and used by so many spree killers. Purely coincidental. It's a high capacity, fast-action semi-automatic (not all semi-automatics are created equal, and the AR15 allows a practitioner to achieve automatic-rate fire) that is military built to empty clips effortlessly. Totally the same as a hunting rifle.
Meh. A .223 is still a relatively low-powered round in the grand scheme of things.
And then you point out a MASSIVE bullet used by almost no one but in bolt action rifles. A bullet that is essentially never used in the commission of murders.
Yes, and that bullet is relatively low-powered compared to a 120mm M1 KE round. Which is completely meaningless patter.
It's purely coincidental that it is a weapon enamoured and used by so many spree killers.
May be. Spree killings are actually fairly uncommon, so trying to draw any inferences from such a small data set is fraught with risk.
Totally the same as a hunting rifle.
Nobody said it was. But I, and many others, refute any assertion that an AR-15 is especially deadly compared to many (most) other commonly available civilian semiautomatic rifles.
And then you point out a MASSIVE bullet used by almost no one but in bolt action rifles. A bullet that is essentially never used in the commission of murders.
The point is that .223 is not an especially lethal round. Bringing up something like a 120mm mortar round is silly... 30-06 is a commonly used, generic-as-can-be round, which sits in boxes and boxes in stores and houses all around the USA. And it is a more "lethal" round than .223, but the anti-gun fringe jump all over .223 and the AR-15 to evoke an emotional reaction. It's pure fear-mongering and appeal to emotion.
After all, it's not just a "gun" it's "a high power, fast-action, semi-automatic assault weapon"! Which one sounds scarier and is more likely to get people all riled up? FSM forbid that the thing might even have one of those evil pistol grips or a bayonet lug, or even gasp be black...
The .223/5.56/variations is the round of choice for military units around the world. For killing/maiming people.
It's powerful (dramatically more powerful than a .22), relatively light, and can be loaded in high capacity magazines.
Just to be clear... I'm not saying that you can't kill somebody with a firearm chambered for .223. Of course it's potentially lethal. Pretty much all firearm rounds are potentially lethal, even something like .22 Short rounds.
What I'm saying, is that the .223 is not particularly more dangerous than other common rounds, to the point that there's any reason to single it out for special attention. And the fact that military forces choose it doesn't dispute that. There are a LOT of reasons why military forces make the choices they do, and they're as likely to be economic forces as purely strategic ones.
When the military chooses it as the round of choice for killing people, it's pretty nonsensical to try to hold it as some sort of weakling.
If sheer lethal effectiveness were the only criteria used to select a round, .223 would not be the first choice for killing a human being.
Of course you can make any point with relative comparisons. Compared to a bb and a slingshot, .223 is deadly-as-fuck. But compared to many other rounds that you can commonly find firearms chambered for, it's average at best.
The point is, like you said, it's exceedingly rare.