I'm going to hope that this is just an unintentional lack of knowledge on your end. If you're actually open to reading facts, please keep reading. Otherwise, feel free to ignore this.
First, in response to the comment about people that keep "shooting up schools and other public places with automatic weapons." This is incorrect. The phrase "automatic weapon" refers to a weapon that can discharge more than one projectile due to a single action (pulling a trigger or actuating some other mechanism). As far as crimes with automatic weapons go, they are so low as to be lost in the underflow of the number of other assaults. In 1934, the National Firearms Act regulated automatic weapons, suppressors, short barreled rifles and shotguns, and "other weapons". Since that date, you have to pay a $200 tax just to be allowed to purchase the weapon. You also have to undergo a background check even more thorough than most class 3 Federal Firearms Dealers. The automatic weapon must be registered and kept at a known location at all times, and the Feds can knock on your door at any time of the day or night and demand that you produce that weapon immediately for their inspection. If you can't, it's a federal felony.
Since 1934, there have been 2 murders committed with registered automatic weapons. As far as unregistered automatic weapons go, numbers vary, but are again so low as to be statistically insignificant. According to GunCite ( http://www.guncite.com/gun_con... ), 4 police officers were killed between 1983 and 1992. And even when targeting groups that are thought to have large numbers of automatic weapons, virtually none of the firearms recovered in raids on drug houses, gangs, and so on were automatic. For all intents and purposes, automatic weapons are not used in crime.
And, since 1986 when the NFA was amended, only automatic weapons made before that date are now available for purchase by the public. This amendment ended out pricing most automatic weapons out of the reach of the standard consumer, and for those that do buy them, they're usually purchased as investments not, not with the intent to shoot them.
As far as the second half of your comment goes, I'm going to assume you're talking about so called "assault weapons", or what are more accurately termed modern sporting rifles (MSRs). And when people think of an MSR, they think generally of an AR-15 variant (go Google what the AR in AR-15 stands for. Hint: it does NOT stand for Automatic Rifle). What is so bad about them?
1: You say "their only real purpose seems to be for killing lots of people efficiently." First, the caliber of most AR-15 variants (5.56x45 NATO, or .223 Remington (and yes, the specs are not completely equivalent between those to calibers, but for the sake of argument, we'll assume they are)) is small enough (and fast enough) that the rounds tend to not do all that much damage to a man-sized target. In fact, in many states, it's illegal to hunt deer with a .223, as it's likely to only wound and not kill it. So, no, an AR-15 is not a particularly efficient killing machine. If you don't believe me on this, go find an Iraq/Afghanistan vet. If they're willing to talk to you about their experiences, ask them about how effective the M-4 was at killing the enemy. Or use Google. The stories are out there. The only reason the US Military uses 5.56x45 instead of 7.62x54 (the old .308 Springfield cartridge that got your (great)grandfather through World War II) is that you can carry 70% more 5.56 than you can 7.62 for the same weight and size of package.
2: Because the AR-15 platform is so modular, my wife and I can shoot the same rifle. My arms are a little shorter than hers. I can adjust the stock. Because it has a pistol grip, I can hold it more comfortably. If you take a look at the definition of an assault rifle from the 1994 US ban, it involved a rifle that could accept a detachable magazine and had 2 or more of any of the following attributes:
- Folding or telescoping stock
- A pistol grip
- A bayonet mount
- A flash suppressor
- A grenade launcher
We've addressed why the telescoping or folding stock is a good idea. The same with the pistol grip (trying to wrap your hand around the stock and still reach the trigger is much less comfortable and natural than using a pistol grip). Including bayonet mounts was silly. Bayonets aren't "shot" out of the end of a rifle, and never have been. The only way a bayonet could even do any damage is if the attacker had taken the time to actually buy and mount the bayonet on the lug, and then get within arm's reach of his victims. Flash suppressors really are misnamed. They can help hide the muzzle flash of the rifle, but their primary purpose on any sort of MSR is to reduce recoil and keep dust from kicking up if you're shooting while laying down on your belly. They do not add or detract from the lethality of the firearm one bit. As far as grenade launchers go, they really don't matter. If you take a look at the National Firearms Act, it's illegal to own any grenades to shoot out of your grenade launcher even if you had one.
So, looking at the criteria for defining an "assault weapon", there's not one item in the list (other than POSSIBLY the detachable magazine) that makes the rifle any more lethal than the lever action rifles common over 100 years ago.
And one last thing: the comment about automatic weapons "not existing when the constitution was written", you're right. They didn't (although there were experiments, and I think the first patents for the first automatic weapons were granted not very long after the Constitution was approved). Think on this, though: the first two battles of the American Revolution were the Battles of Lexington and Concord. Those battles were fought because British General Thomas Gage was concerned that independence-minded colonists were going to use their privately owned artillery (yes, private citizens and militias owned cannons back in those days). And the Founders, with that thought fresh in their minds, explicitly amended out Constitution to prevent any such orders from ever being given again.
If you really think that "assault weapons" should be outlawed because they didn't exist when the 2nd Amendment was ratified, are you willing to get rid of your TV, cell phone, tablet and Internet connection because they didn't exist when 1st Amendment was ratified as well?