During one of last night's MSNBC chat shows, one of the pundits (failing memory says Chris Matthews) said something like:
When they wrote the Second Amendment, they were talking about muskets!!
Which is true. Here's the text
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.
In historical context, of course, a Militia in 1791 was the same as the National Guard today: a state entity, not insurgents in Montana or other shitkicker empty states. That's the main reason opponents of unfettered access to guns say the 2nd has no basis for civilian access to military grade weapons.
Look back at 1791, and you see that (esp. non-urban) American civilians and standing army soldiers used the same weapon: a musket. Yes, the army had cannon and ships and such, but the soldier and the civilian had the same weapon. While I don't follow the NRA at all, I expect that someplace they've made just that argument: the civilian deserves access to the same weapon as the soldier, or the army will subjugate the populace.