1) "Assault rifles" are not a thing. Certain specific firearms were banned by a piece of legislation colloquially known as the "Federal Assault Weapons Ban," but that doesn't make it a thing any more than the PATRIOT ACT makes it patriotic to eavesdrop. It was passed in 1994 and expired in 2004.
2) California has its own wacky gun laws. I don't know or care what they are.
3) The thing you're talking about is the also ironically-named "Firearm Owners Protection Act" of 1986, and it sort of banned machine guns, but only those manufactured after passage. It's still perfectly legal to buy/own a machine gun built before 1986.
4) In fact, the FOPA did not ban machine guns; it simply requires people to pay a one-time Federal Excise Tax fee. Search for "how to buy a class 3 firearm," for details. The only real hurdle is sometimes getting approval from the local authorities, and it may require some persistence, unless you have a criminal record, and then you can forget about it.
State law may vary. IANAL, and this is not legal advice, etc...