"If gun laws in fact worked, the sponsors of this type of legislation should have no difficulty drawing upon long lists of examples of crime rates reduced by such legislation. That they cannot do so after a century and a half of trying — that they must sweep under the rug the southern attempts at gun control in the 1870-1910 period, the northeastern attempts in the 1920-1939 period, the attempts at both Federal and State levels in 1965-1976 — establishes the repeated, complete and inevitable failure of gun laws to control serious crime."