Second of all, assault rifles, machine guns, and sub-machine guns have all been illegal in the United States since 1934. Anybody that owns one must go through a rigorous background check, fingerprinting, licensing, registering ... not to mention the fact that fully-automatic firearms are expensive as hell. Like, ten or twenty grand, expensive.
TWO murders in the United States have been committed with legally-owned fully-automatic firearms since 1934. One was by a Law Enforcement Officer. Both were after the 1986 law that put even more restrictions on them.
And last ... The Second Amendment protects the rights of civilians to own the same type of arms the federal government owns. (Most supporters of firearm rights wouldn't argue for rights to this extent, but I will display an accurate understanding of the Constitution.) When it was passed, it protected the same types of firearms. The argument that the Founding Fathers could never imagine the types of firearms we have today is not only irrelevant (because, as pointed out, they protected the same types of firearms for everybody), but also ignorant (unless we think that nobody ever expected technology to advance).
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