"Why do you need an AR15?"

[stanc]

It is revealing that in the thousands of pages of proceedings that were published in the course of the debates that took place in the state and federal legislatures before the drafting of the bill of rights and throughout the ratification period, little mention is made of the individual right to keep and bear arms. This indicates, I think it is fair to say, that whatever their disagreements about the inclusion of a bill of rights in the Constitution, the Federalists and Anti-federalists were unanimous in their support of an individual right to keep and bear arms. They were also unanimous in assuming that "the right of the people to keep…arms" included the individual right to keep ordinary personal arms for armed self- and community-defense, especially against burglars, robbers, and rapists.

I think you are assuming something not supported by the evidence. (See below.)

This lack of understanding of the nature of the controversy between the Federalists and the Anti-federalists is one of the two principal reasons, I believe, why the proponents of gun control have reached the erroneous conclusion that the Second Amendment guarantees only a "collective right" or "states' right" to keep and bear arms.

Actually, there is good reason to conclude that the Second Amendment only guarantees the right (of state militias) to have arms for collective defense: It specifically addresses state security, but makes no mention of keeping and bearing arms for individual self defense.

States' RKBA that preceded the 2nd Amendment:

North Carolina (1776) "...the people have a right to bear arms, for the defence of the State..."
Pennsylvania (1776) "...the people have a right to bear arms for the defence of themselves and the state..."
Virginia (1776) "...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..."
Vermont (1777) "...the people have a right to bear arms for the defence of themselves and the State..."
Massachusetts (1780) "The people have a right to keep and to bear arms for the common defence."

If the 2nd Amendment was meant to be an individual right, why did it not follow the example of the Pennsylvania and Vermont constitutions, and explicitly cite self defense as a reason, instead of only defense of the state?

Considering what the 2nd Amendment actually says, and taking it in context with the Militia Act of 1792, I have to agree with the following comment that I read elsewhere:
“It is therefore entirely sensible that the Second Amendment’s prefatory clause announces the purpose for which the right was codified: to prevent elimination of the militia.”