Since I am afraid this thread will potentially dissolve into an argument, I offer no opinion, and just want to say something that is relevant to the discussion, perticularly regarding the phrase 'Assault Weapon'.
I believe the laws in the US are, at least on a federal level;
If one is over the age of 18, having no prior felony convictions and having no history of mental illness requiring a court ordered intervention, they are allowed to purchase or own any long gun.
If they are over the age of 21, with the same rules regarding felony convictions and mental illness, they can purchase or own any handgun.
There are more restrictions, for example certain types of weapons (explosives, automatic weapons, silenced weapons, things like that) are illegal unless they are registered. One needs to pay for a government tax stamp to get them registered, and the registry for automatic weapons was closed in 1986, so effectively the average civilian can't get their hands on automatic weapons
*. It is entirely legal to buy an assault rifle
registered (not made) before 1986, but due to the fact that they are rare (and getting rarer), and they are mostly in the hands of collectors who won't sell them, getting even the cheapest of automatic weapons would cost so much money that the average person would never be able to get one unless they saved up for a long time. Impulse buyings just don't happen, and as such if a spree killer ever uses an automatic weapon you could bet money on it that it was illegally modified.
* The point of is post is to clean up some confusion that I have noticed is quite common. This paragraph is not relevant to my personal stance on gun control, it is just a result of the fact that I dislike when people intentionally misrepresent information. It has been pointed out before on various websites that the media in the US often uses incorrect terminology when describing gun laws, which can lead to confusion. After seeing some examples, I can't help but agree that it does seem purposeful at times.
One thing that seems to be quite common, is that people often refer to shooters using 'assault weapons', and saying that these military weapons should not be legal to own. 'Assault Weapon' is not a real term, but is similar to the term 'Assault Rifle'. An Assault Rifle is a lightweight,
select-fire weapon with a detachable magazine, chambered in an intermediate cartridge. The important point here being that they are select-fire, meaning that they can be fired in automatic (or burst) mode. A lot of people don't know the specific definition of an assault rifle, but they do know that they are military type, automatic weapons. So the fact that people are referring to 'Assault Weapons' can be confusing, and infers that the shooter had an automatic weapon.
Generally when the term Assault Weapon is used, it is used to refer to a 'military style' weapon, or in other words a civilian variant of a military weapon. While it is understandable that one may see these as unreasonable for a civilian to own, it is an important point that they are invariably semi-automatic only. As such, they have no more capability than a lot of other hunting weapons that one may buy. In fact, since Assault Rifles (and by extension, civilian variants) fire an intermediate cartridge, they are in fact less powerful than most hunting weapons, which tend to fire full-sized cartridges.