The second amendment was passed at a time when canons, muskets, and bayonets were some of the most powerful weaponry on land. Excepting the artillery, all this could be purchased and owned by any sane crime-free adult at any time. Hence, the people were entitled to more or less the same level of armament as the military.
The amendment is unclear as to how "Arms" could apply to the 21st century. When we're talking about "Arms," are we talking about guns? If so, what guns? What about grenades? Tanks? Helicopters? There is no clear answer in the amendment's language, but 10,000+ lives are lost every year we pretend that firearms have a place in a peaceful, modern society.
We don't need the guns anymore. Big Brother is not coming for you. Please stop the killing. Repeal the second amendment.