Mr President could fix this with a simple Executive Order during National Emergencies and Override ALL State Gun Laws, and then Threaten to Criminally Prosecute them under US 18, 242 Deprivation of Rights.
Section 242 of Title 18 makes it a crime for a person acting under color of any law to willfully deprive a person of a right or privilege protected by the Constitution or laws of the United States
42 USC 1983 addresses that quite well.