That's a definite no. Under GCA '68, it's a felony to buy or sell a rifle across state lines unless you have a FFL (Federal Firearms License). Any time you buy a gun from a dealer, they need ID to make sure you're a resident of that state, in addition to performing background checks. If you want to buy a gun from out of state legally, whether from a FFL dealer or a private citizen, it has to be shipped to a FFL in your state, then you have to go to that dealer and do the background check and 4473 form just as if the gun was in that dealer's inventory.
Of course, there's always ways to get around it, but if you do, both the buyer and seller are breaking several laws.
But what's the point anyways? The legal availability of these weapons doesn't have any effect on crime. Also, a true assault rifle is select-fire, and therefore a Class 3 weapon. Getting one requires, among other things, fingerprints, passport photos, a sign-off from the local police chief, a wait of several months while the FBI checks out your background, and a $200 fee. I'm sure that the whole process is illegal in California anyways.