"Why do you need an AR15?"

[stanc]

Montana and others arguing that we don’t need a particular item misses the point
In my view, the 2A establishes the need, even though it is not immediately apparent to most.

The only need addressed in the 2A is access to weapons for arming the militia. However, that's been rendered somewhat moot, because those state militias that still exist, either are unarmed, or issue required weapons. There is no longer a need for individuals to provide their own weapons; and in fact, modern military forces typically do not permit the use of privately-owned weapons.

The need for a defensive weapon is akin to the ejection seat in combat aircraft. One doesn’t “need it” until I need it right now!

Precisely. And the only type of firearm practical for "RIGHT NOW!" accessibility on a 24-hour, wherever-you-are basis is a pistol or revolver.