100 rounds is pretty low. Back in the day I carried 6+1, six mags in my pouches and one in the M16. Depending on the mags issued that was quite a bit of ammo. While many never beens and one way range rangers make a thing about ammo weight, soldiers don't. You carry what you are issued, no real thought to the weight, your life depends on it so you hump it.
Most military ARs used by grunts these days are equipped with optics. Iron sights are good but not as versatile. I resisted dropping irons, but these days I would recommend optics, even simple red dots over irons.
The military trains combat soldiers on tactical ranges measured in meters, not yards. Yard ranges are for competitions geared toward the civilian side of shooting. Pop-up targets so engagement times are limited to try and simulate the real world better than hoist and paste. Ask combat soldiers just how realistic the stress and timing is of the pop-up system and most would laugh, rarely does the enemy expose himself long enough or steady enough to develop a good sight picture. (Civilians are a whole 'nother story, which is why most experienced soldiers smile when one way range rangers prattle on about armed citizens keeping the gubmint in check.) Last I checked the Army trains out to 300m with the M4 for quals. 600 yards is what Army snipers (7.62x51) train to, about the only Army soldiers still using yards.
Anyway, the threat will not be far off and let you know he is the one in the crowd you need to sight in on like a one way range. He or she will be mixed in the mass of survivors trying to walk out with you. (If you think you will safe and comfy at home when a natural disaster odds are you will be at work or on the road) Even if you are at home you still have to stay awake and have 360* over watch. If the 'masses' swarm you can get all but one, and still have a losing proposition.
Jerry probably has the best idea, not a lone wolf with one weapon but a group holding a small area where survivors and the injured can find refuge until outside help arrives.
My weapon of choice would be my AK. I only own the one, only use the one. Had it for over 2 decades and used it in all kinds of weather. It is an extension of me. Unless the badguys develop a huge case of stupid they won't line up for one way range well placed shots, so tactics trump X's. I doubt there will be too many duels
That said I've carried the Mattel toy enough that to this day I pick one up as if I never laid it down. My advice would be pick one and drill, dry fire, carry it around until you forget you have it and can deploy it without any thought to how you do that. Put dummy rounds in the mag to do immediate action drills to clear a stoppage. Run your mags hard, study what causes them to fail and maintain those feed lips. When the 30 round mags were first issued to us the feed lips were the weak link so we soon started carrying pliers to bend them back in, many 'popular' which means cheap or overhyped in the gun rags, magazines can have thin or weak feed lips.
If you plan on running lone wolf style with only one weapon if a disaster hits- good luck
