First off, a SHTF weapon should be as general purpose as possible, you are not using it for knocking down doors and clearing houses. The AR-15 is dependent on velocity for maximum effectiveness, so you want a long barrel to maximize velocity.
Second, you need to keep the weapon working so you need a butt-load of spare parts. Brownells probably won't be delivering after TSHTF. I recommend at least 2 full trigger groups, 2 complete bolt-heads, 5 complete spring-sets and extra stocks, handguards, pistolgrips, CHARGING HANDLES (they get bent up more than most people realize) and anything else you can get. several extractors and firing pins above and beyond the bolt-heads are necessary as well.
Third, AR mags suck! Like beer cans, they are not meant to be reused over long periods of time (just one more strike against it IMHO) so you need a butt-load of them too, 50 at least, if you want to keep it working. Yes I know some mags are better than average, but as a general rule my statement stands.
Fourth, in my own view, and semi-automatic carbine with an intermediate round, should be relegated to retreat defense and use by those who cannot use a full power rifle. As such, I would make it standard A2 configuration with good iron-sights and the only optics I would consider would be an Aimpoint, ACOG or Elcan, nothing else is robust enough to handle extended use without support and only the aimpoint has a long enough battery life to consider in a situation where batteries are likely to be rare or unavailable.
Fifth, lots and LOTS of ammo! If you have a bean shooter, be ready to throw lots of beans.
Lastly, sell it and buy an AK! You will need lots fewer spare parts, it will run dirtier and more neglected than any AR, and it fire something that you can kill a decent sized animal with.
After TSHTF you will find that your rifle will be far more important for killing food and pests than people and while no Semi-automatic carbine is ideal for hunting larger game, the 7.62x39 is far better suited for it than any 5.56. Likewise, when you go out in the field for any reason except when you are specifically hunting, you will be carrying your defensive weapon rather than a hunting weapon. After TSHTF you will need to kill food animals when you CAN. If Bambi or Bullwinkel hop out in front of you you need to bag him, because, as every hunter knows, they are never around when you are looking for them.
My primary weapon for SHTF use is a full sized General Purpose Battle Rifle (H&K 91, CETME, M-1, M1A, etc) because they are good for something other than spraying a lot of bullets and hoping to kill what you shoot at. If I hit an Elk it will almost definitely be going into my freezer (or smokehouse) and without tracking it all day or requiring riddling its meat with bullets or trying for a head shot. Likewise, if Mad-Max shows up with his semi-armored post-apocalyptic V-8, I'm gonna riddle his ass with holes, not muss up his paint-job, to say nothing of the guy hiding behind a lodge-pole pine.