Regulate the militia.

The second amendment states
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.The key here is a well regulated militia. If a citizen wants to own a firearm he should be required to undergo
1. A background check
2. Be required to undergo safety training
3. Undergo periodical mental health exams
4. be required to keep any automatic weapons in an armory