In my opinion the main purpose of the second amendment was to secure the peoples right to protect themselves from tyranny. The writers of the Constitution understood that people posessing firearms was vital to securing their freedom in any case, and when the government took them it was all over. A people can lobby and protest and vote and do whatever they can to resist tyranny, but when firearms are taken and the government/military still has them, then there is absolutely nothing a people can do to get rid of that government, its the final line pretty much.
I think that is why they wrote it as 'shall not be infringed'. They expected times to change as well as the weapons, but the principle will never change. The Nazis banned all but single shot rifles and shotguns. Compare that to a military with 5 round capacity bolt action rifles and sub machine guns. The civilians if they ever were to rise up against their tyrannical government, might as well not have any guns. Same principle here in America today. If it ever became necessary for people to fight a tyrannical government and their 'assault weapons' had been confiscated, then bolt action rifles and semi auto hunting rifles with 5 round mags don't really fare well against real assault rifles and machine guns.
The revolution was still fresh in the writers' minds, and I'm sure they were thinking of every way possible to keep the country free in the future.