[Theohazard]

Because you're building the rifle for a SHTF situation, I think you should go with a 16" standard-profile chrome-lined barrel with a mid-length gas system and M4 feed ramps. It doesn't matter that much which manufacturer as long as it's a good one (BCM, Daniel Defense, Noveske, Colt, LMT, etc.).

I say 16" because a shorter barrel can sure come in handy, and all you get from a longer barrel is a little bit more velocity and a longer sight radius, and you can still give yourself a long sight radius by using a longer rail.

Chrome-lined barrels are slightly less accurate than stainless, but they're more durable and more corrosion resistant, and I think that matters more for your intended needs.

I'm a big fan of a mid-length gas system for a 16" barrel. Most 16" rifles running a carbine-length system are over-gassed; with a mid-length you get less gas pressure so extraction is better and recoil is less.

I don't recommend a heavy barrel on a rifle you might end up carrying for any length of time. Heck, you might even want to go with a thin-profile barrel (though that would lower service life a bit). It's amazing how heavy just a few extra ounces feels after carrying a rifle around for only a couple of hours, and on a long patrol it gets way worse.

I agree that M4 feed ramps won't hurt, and since you've decided to go with a flat-top receiver availability won't be a problem at all.

As for the BCG, just go with a good standard mil-spec one; that's what I've always used and I've never had a problem. But if someone has experience with the chromed or nickel-boron ones and can convince you they're noticeably better, then go for it.

If you're planning on not running any optics right away, make sure you have some good iron sights (I like Troy, but there are a lot of good ones out there).

I'd also recommend a rail, I like a 12" - 13.5" free-float on a 16" barrel, but if you don't need the extra length you can save a lot of weight by going shorter. And as nice and as light as the smooth-profile modular rails are, traditional quad rails are better for a SHTF situation because there's no separate rail panels to lose or break. Also, you don't need to get rid of your standard front sight/gas block if you don't want to. There are plenty of long free-float rails that accommodate a standard front sight.

As for the buffer system, I like the VLTOR A5; it's longer and more forgiving with various-pressure ammo. But it's not completely necessary with a mid-length gas system; just use a heavier buffer and you'll have very similar results to the VLTOR A5.

I also recommend a good trigger, that can make all the difference in accuracy. Make sure it's a non-adjustable two-stage; my favorite is the Geissle SSA-E.

That's the main stuff I could think of for now. Feel free to ask anything else in case I left something out or you want more detail. And I'm definitely not an AR-15 expert, so I'd welcome any input from other members.