The purpose of the 2nd Amendment is that the citizens, AKA the militia, can successfully thwart the ambitions of tyrants who would seek to take over our country or restrict our rights.
The way to thwart such attempts is to have the citizens armed THE SAME AS the armies of such would-be tyrants. It's hard to imagine our wonderful men and women in the military turning against US, but who knows how it would be presented to them--especially with a Clintonesque administration of the future (with the corresponding selections of commanding officers, etc.).
Bottom line: we get the SAME weapons soldiers get. Hint, they aren't packing black powder, single shot rifles as were the army and the citizens of Revolutionary times. The details--the changing specifics--are not static. The purpose is.