"Why do you need an AR15?"

[stanc]

Only one person created the Second Amendment. http://www.billofrightsinstitute.org...ill-of-rights/

From your source, emphasis added:

The first 10 amendments to the Constitution make up the Bill of Rights. Written by James Madison in response to calls from several states for greater constitutional protection for individual liberties, the Bill of Rights lists specific prohibitions on governmental power. The Virginia Declaration of Rights, written by George Mason, strongly influenced Madison.

So it wasn't just thought up/created by one man as you infer.

"Calls" for greater protection do not equate to being co-author/creator. Madison alone wrote the Bill of Rights, although he clearly was strongly influenced by Mason.

From the Virginia Declaration of Rights:

...a well regulated militia, composed of the body of the people, trained to arms, is the proper, natural, and safe defence of a free state...

Note that, like Madison did with the Second Amendment, Mason only addressed collective defense.