Okay, so most of us can agree on two important things: Some weapons or weapon features or accessories should be banned, and some should not. This means the real question is, where to draw the line? What is the principle? This is a very important question, and I've thought about it a lot. The answer I've come up with was heavily influenced by American tort law. Case law has evolved organically under the guidance of some very smart, well-meaning people who have guided it while in full-frontal contact with reality. They are dealing with cases, not their own fantasies. So it deserves to be taken seriously.
There is an important distinction to be made between the abnormally hazardous and that which is normally hazardous. I think a related idea applies to weapons, and with similar reasoning. Some weapons cannot be used safely. And since your neighbors are not being compensated, by means of some economic good being made available to them, for the risk they are subjected to, there is no reason they should put up with it. That means you can't have nuclear weapons, bazookas, hand grenades, incendiary grenades, or a machine gun.
Notice, and this is very important, that a modern gun, as such, is not (to coin a phrase paralleling the above legal one) an ultra-hazardous weapon. In fact, it is in one way the opposite of that. If used at the range at which it was intended to shoot, a modern firearm, in contrast to the old muskets and flintlock pistols, is a precision instrument. I like to tell my students that if I had a good pistol I could dot the "i" in that exit sign at the other end of the lecture hall. (Some of them look startled.)
An AR-15 is a precision instrument and not an ultra-hazardous weapon, in my sense. It is not a machine gun.
This brings me to the reason I put "assault weapons" in scare quotes, above. True assault rifles were invented for the military during WWII (in Germany and Russia). The military definition is: (a) a medium range rifle that (b) has a selector that switches the weapon between fully automatic and semi-automatic. Select "fully" and it becomes a machine gun.
The latter clause is why the AR-15 (pictured above) such as the one the Sandy Hook murderer used, does not fit the military definition of an assault rifle. It lacks this switch and only has the semi-automatic mode of functioning.
What "semi-automatic" means is this. When you fire a bullet, the mechanism uses some of the energy expended to do three things: expel the spent cartridge, move a new bullet into the chamber, and cock the firing pin (this is called cycling). To fire, all you need to do is pull the trigger (this is called single action trigger).
In a semiauto weapon, only the cycling is automatic. The firing is not: ie., the gun does not keep cycling and firing, both, if you just hold the trigger down.
A typical revolver, like the S & W .38 I inherited from my Dad, accomplishes somewhat the same effect of a semiauto by means of a double action trigger: as I pull the trigger, it rotates the cylinder, moving the spent cartridge out of the way, presenting a fresh round to the firing pin, and cocking it. (I just timed myself and got up to four shots per second. The gun was unloaded, of course.) Doing all that work with your trigger finger makes the weapon less accurate, unless you have a really fine, smooth-shooting revolver like the Colt Python, but that would run you around $1,500.
Hm. Maybe you'd rather have a semi-automatic pistol, like the classic Colt 1911 (named for the year it became standard issue in the US Army). This highly esteemed weapon is just as automatic as the AR-15, except for one thing: before the first shot you fire, you must cock it manually by pulling back the slide. Thereafter it cycles automatically.
Or, if you want to avoid that one little hitch, you might try one of the many kinds of semiauto pistols where the trigger is double action on the first shot, single action thereafter, and auto cylcing throughout. These weapons are exactly as automatic as the AR-15.
Now you can probably guess what my point is: These "automatic" weapons have been extremely common for a long time. They have been standard military issue for a century. They are carried by virtually all armed police officers. There are millions and millions of them in the possession of civilians who use them for entirely legitimate purposes.
As a matter of fact, they are the weapon of choice of mass murderers, overwhelmingly preferred to weapons like the AR-15.
It's easy to see why. These weapons, as I have said, are made for medium range shooting: shooting at enemy coming over the next hill. Not for long range sniping, and not for shooting somebody in the same room with you. Hell, it has a 20-inch barrel, for medium range accuracy. What do you need that for, when your victim is in the same room with you? You've accepted the weapon's bulkiness, clumsiness (at close quarters), and conspicuousness, and gotten nothing in return. You are better off with a 1911 and a bag of loaded magazines over your shoulder.
[I don't mean this in a crass way. Innocent lives are at stake and to figure out what we should do about it we have to take no-BS way about what these demented dirtbags are actually going to do.]
The "automaticness" of these "automatic" weapons does not make them "ultrahazardous," in the sense in which that could justify banning them, and they give the mass murderer no advantage they couldn't get from weapons that are extremely common and obviously legitimate. It is no reason to ban them as a response to mass shootings. None.
There actually is another issue here that I have not touched on. There is one difference between AR-type weapons and pistols like the 1911: magazine size. The magazine in Lanza's AR may have held as many rounds as 30 (the max allowed in the state of CT). Magazine size of a 1911: 6. Is that a reason to limit the size of magazines (in effect, banning all larger than the limit)?
I'll try to post about that later.