A landmark Federal Law is passed. All guns of any kind in private hands are banned. Legal gun owners are required to turn in their guns or face jail. Being law-abiding citizens, they do so. Citizens who do not cooperate are jailed.
Neighbors turn in neighbors who are suspected of posessing illegal guns.
Children, at the urging of the schools, turn in their parents. Police departments, already stretched thin, must dedicate additional resources to the enforcement of these new laws. As with drug and alcohol prohibition, a huge black market for illegal firearms spawns a new, more sophisticated network of organized crime. The illegal gun market explodes. Gun smuggling and black-market gun sales are now really big business.