You're correct that a law passed today cannot make you a criminal for owning guns yesterday. But a new law can outlaw that gun, and can either make you turn it in, with or without fair compensation from the government, as well as impose ridiculous fees and other restrictions, with the failure to comply with those new restrictions made a new crime. I am old enough to remember when LSD was not illegal to manufacture or possess, and then, by government fiat, it was added to the list of illegal drugs. Possession prior to the reclassification as illegal was not a defense if you were caught in possession of the drug. The exact same thing can happen with various types of classes of guns. In addition to so-called assault weapons, I think all semi-auto rifles and shotguns will follow shortly thereafter, followed first by outlawing semi-auto handguns, and ultimately banning all privately owned handguns. . My own family, including a son who is an LEO think I am an alarmist but once lost, freedom is very hard t regain. I would rather be a bit paranoid now instead of sorry later.