AR platform hands down! SHTF who will care if it is restricted anyways. I have one SHTF gun it is an old school Olympic Arms with a 11.5" barrel. I know many people will say oh you need a gun that reaches out there but I can say anyone that moves around in daylight in open area will be out of the game quickly. Most survivors will stay under cover and most fights will be under 150 yards anyways most likely around 50 to 75 yards in urban or bush settings.
7.62? Well if you can only carry 200 rounds your expensive platform will be a poorly balanced club when you run out of ammo.
5.56 seems the best compromise for round count over power in closer encounters. Ever ask why use a noisey 5.56 round when you need to eat something like a rabbit instead of a .22 rimfire round. So I can easily carry 300 ronds of 5.56 and another 2000 rounds of .22 rimfire. Hmm any 300.00 rimfire conversion kits out here for non AR platforms, don't think so.
Is a .22 rimfire a stopper, not really but would you rather use your empty semi auto as a club or swap over to 26 rounds of Stingers with a conversion kit?
I will stay with my 20 year old Oly it always goes bang and groups very well and my CMMG .22 conversion kit just chugs along as well, at the end of the day it is all about choices and flexibility, as most decent rifles are already accurate and dependable. And forget about lasers, flashlights, red dots, as batteries run out....
Just practise with you irons and get a simple scope that works well.
If you choose a .22 as your SHTF gun get a SR 22 or 1022, here is my 10/22 with a 10" charger barrel and all that useless battery powered gizmos that would just come off when the batteries fail, at least I have a scope and great backup irons.
Just my .02 cents.
Randy