Budget TBD. Skinwalkers talk aside, this is an exercise in learning about light weight AR-15 that would be purposed for self/home defense (officially - as a range gun practically) and that would be light enough to carry a bit.
In my original scenario, the "hero" was an archeaologist who drove his vehicle as far as it would go up a canyon and then hiked maybe 2 or 3 moderate miles farther up to where the ruins were. For me, that sort of trek would be perfectly reasonable to assume I'd undertake and in that scenario, I'd want the option of a light weight rifle, 90 rnds of ammo, a knife, and water.
In case anybody is worried, this type of thing is about as close to "fantasy" of you know what hitting the fan that I even think about. I do have a bug out bag but that's for tornados which, in Indiana are a real threat. We are the number 3 worst state in the Union for tornados (OK is the worst, forget the second).
So summing up this interesting thread, my next AR will be all about light weight...as light as I can get it. I won't bother with SBR but thanks for the advice. That Colt RC pointed out really caught my fancy. The Bushmaster carbons are fascinating too but, back to the skinwalkers, they live in dusty desert regions and I'd like a dust cover.