You cannot buy an AK-47 in the United States. With the exception of a select few people, they are banned. This also goes for M-16's, Uzis, and anything else that gets lumped into the category of "Assault weapons". (Pointless category, a damned knife used to stab someone is an "Assault Weapon")

You cannot buy so-called "Armor piercing" ammunition in the United States.

You cannot buy a firearm in the United States if you have been convicted of a Felony. Depending on the offense, there are also misdemeanors which bar sales of firearms and require the confiscation of firearms if you have possession prior to the offense, most notably Domestic Violence of any sort. (Wanna have fun with your gun owning target shooting ex? File a restraining order request. Your ex is now no longer allowed to have firearms, regardless of whether or not he/she actually DID anything)

You cannot buy a firearm in the United States if you have a history of mental illness. (Oops, congressionally mandated loophole there)

There are age restrictions, locale restrictions (I.E. no carrying in places where alcohol is sold for consumption), the list of gun restrictions is endless.

Why in hell do we need more laws when the police and the criminal justice system CANNOT ENFORCE THE LAWS ALREADY ON THE BOOKS?