Please. Explain.
Well, I'll start with a quote, so we're sure we're on the same page as to the wording.
Second Amendment said: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.
This, by my reading, protects and individual right to keep and carry guns, knives, swords, or whatever weapons are available. It also provides one rationale for the protection of this right, the formation of militia. Others include an intrinsic right to self-defense, and defense of one's loved ones and property. These are natural rights not explicitly enumerated by the Constitution, that nonetheless exist.
The government is also empowered to do this:
The Constitution-Article I said:[The Congress shall have power] To regulate Commerce with foreign Nations, and among the several States,
in order to do this:
The Constitution-Preamble said:insure domestic Tranquility,
So, in cases where the government can prove an overwhelming interest in regulating interstate commerce in a way that ensures domestic tranquility, it can make sure that people buying machine guns aren't violent felons.