How do you arrive at the conclusion that there are privately held semi-automatic weapons that are unlawful at the state and federal level? There was a Clinton era ban on some configurations of grips and handles for semi-automatic rifles; but that was repealed a long time ago.
On repeat: most privately-owned semi-automatic assault weapons, seemingly the gun group of choice for mass shooters, are considered, on a state and federal level, to be unlawful.
I assure you, semi-automatic rifles of all kinds are lawful at the federal and the state level. Even in California, if you’d like to buy a semi-automatic AR-15 with detachable magazines, you may do so, as long as the stock and grip are all-of-a-piece, without a thumbhole or pistol grip. This is similar to the rule followed by the Clinton era ban, which means that semi-automatic rifles were constantly in circulation during that ban. However, that ban has been repealed. The AR-15 in its classic configuration, and similar service pattern semi-automatic rifles, are fully legal at the federal level. Consequently they are also legal in most states, since most states do not have laws against them.
What is the law you are referring to, and what are the weapons you are referring to, when you say that “privately-owned semi-automatic assault weapons” are unlawful on a state and federal level?