I think the OP is on the right track, but it's actually simpler than that. In 2008 the Supreme Court settled many 2nd amendment issues with DC v Heller. Here are the main points:
The first quote establishes that service in a militia is NOT required to be protected by the 2nd:
1. The Second Amendment protects an individual right to possess a
firearm unconnected with service in a militia, and to use that arm for
traditionally lawful purposes, such as self-defense within the home. (page 1)
In addition, in Heller the court commented on a previous decision, US v Miller which was about the National Firearms Act, which regulated machine guns, short barreled shotguns, etc. This clearly establishes what sort of arms are covered:
(f) None of the Court’s precedents forecloses the Court’s interpretation.
Neither United States v. Cruikshank, 92 U. S. 542, 553, nor
Presser v. Illinois, 116 U. S. 252, 264–265, refutes the individual rights
interpretation. United States v. Miller, 307 U. S. 174, does not
limit the right to keep and bear arms to militia purposes, but rather
limits the type of weapon to which the right applies to those used by
the militia, i.e., those in common use for lawful purposes. (page 2).
Now if there was ever a gun that is in common use for lawful purposes, it's the AR15 that some want to ban. It's exactly
the sort of gun that the second amendment is all about. The proposed ban flies directly in the face of the second amendment in general and Heller in particular.