How can I buy an assault rifle?

If by "assault rifle" you mean a select-fire, magazine-fed automatic rifle, like an M-16 or an AK-47, then the answer is complicated. First off, all types of machine guns are banned in the US as of 1986, meaning you cannot legally buy a machine gun manufactured in the US after 1986 unless you are a professional gun dealer with a level 2 FFL. But, you can legally buy any used machine gun manufactured in the US before 1986, as long as you don't live in any of the states where all machine guns are banned, like California. You will also need to have $10,000 and meet all of the standard criteria as well. Upon purchasing it, you will also need to register it with the police and pay extra taxes.