The strongest argument for gun control is found in the 2nd amendment itself, which reads:

A well regulated militia, being necessary to the security of a free state, the right of the people to keep and bear arms, shall not be infringed.

Read it again:
A well regulated militia, being necessary to the security of a free state, the right of the people to keep and bear arms, shall not be infringed.

What does "well regulated" mean?  In District of Columbia v. Heller (2008), the U.S. Supreme Court stated that
the adjective 'well-regulated' implies nothing more than the imposition of proper discipline and training.

What is the mechanism by which "proper discipline and training" is provided to members of this militia?  How well disciplined was Adam Lanza? James Holmes? Jared Loughner? Cho Seung Hui?