The second amendment states that "the right of the people to keep and bear arms shall not be infringed." It does not limit what weapons "we the people" can have, nor does it give the federal government the power to do so.
These weapons are "necessary to the security of a free State" because the if federal government has access to fully automatic firearms, the people should also have access to them.
The U.S. government should never become more powerful than the people, because throughout history, when government becomes more powerful than the people there is tyranny.