The term "assault rifle" is extremely contentious. Most of the time, it's absurd. An M-16 is an assault rifle, as it's an evolution of the "sturmgewer" term coined in WWII by German to describe their StG-44, designed as an intermediary between overly powerful full-sized cartridge rifles and pistol-caliber submachine guns. Something more controllable under automatic fire, with range effective enough for most soldiers' ability to aim, and with more power and range than a SMG.
But an AR-15 is simply a semi-auto civilian version of the M-16, and I (and many) would argue is not an assault rifle, as it is not intended as a weapon of war. The identical appearance is irrelevant, as is the design of a completely different looking hunting rifle (made to look more classic, for example). A .223 Remington does the same thing out of gun regardless of what the gun looks like. The 1994 Assault Weapons Ban's criteria was based largely on irrelevant and cosmetic criteria. Folding/telescoping stocks, bayonet lugs (seriously?), pistol grip, grenade launcher support, flash suppressors, etc. All it did was to force companies to redesign their stocks/grips and not use flash suppressors. And obviously nobody gave a crap about not having bayonet lugs or lack of being able to launch rifle-fired grenades. None of that changed the fact that a bullet is a bullet and doesn't care how the gun looks that it's being fired from. The ban also didn't include previously manufactured guns, so those were still on the market (at a premium, of course). There's a reason the ban sunsetted after its 10 year term.
That's why I say the term "assault rifle" or "assault weapon" is ridiculous. Is a 00 12 gauge buck shot shell more deadly because it came out of a military styled Benelli compared to a hand-engraved walnut stocked Beretta?
True "assault weapons"? Yeah, you can buy them here. If my memory serves me, 43 states allow Class III weapons. But as others explained, lots of loops to jump through, and it ain't cheap. And those aren't the people that you don't have to worry about. Of course there are other ways; black market, obviously, and anyone with a properly equipped machine shop and the right knowledge can turn a legally-imported semi-auto Bulgarian AK-47 into a fully automatic one (like those guys that robbed that bank in L.A.). It's not magic, it's just mechanics.
The Oregon shooter had legally purchased guns. The same guns that millions of other Americans own and haven't killed and won't kill anybody with. This tragedy, and so many others, took place in "weapons free zones". More, and more restrictive laws aren't the answer. Crazy mofos don't care about those laws. Here in Tucson a nursing student at the U of A killed 3 professors and himself (a "weapons free zone" - don't ask me why nunchucks are on those signs). Also in Tucson, some crazy mofo killed 6 people, and wounded many others, including US Representative Gabby Giffords (the intended targe), in a place that wasn't a "weapons free zone". I think that "weapons free zones" are a bad idea, because they make prime targets for crazy mofos, because the law-abiding gun owners won't be packing there. Like that theater in Colorado that banned guns, and the "Batman" movie opened with a horrible tragedy. I don't have the answer, because banning crazy mofos for being crazy mofos isn't possible. Obviously allowing anybody and everybody to pack is no guarantee of some gunslinging hero to appear to keep the world safe. But I, for one, feel less safe in a so-called "weapons free zone". I don't own a gun myself (yet), but I am more wary of weapon free zones, and consider anyone who thinks these places are safe because of some sign to be complete fools.
