Okay, fair enough questions, here's what I think:
The gas pistons AR platforms are great guns. I have shot many of them, just don't have a privately owned one, yet. Even then, it'll probably be in 5.56 caliber. No doubt the 6.8mm SPC is a much better stopper, but 5.56 ammunition is cheaper, more available, and you have a wide choice of bullet configurations for diverse needs: Full Metal Jacket (Ball); Frangible; Jacketed Hollow Point; Reduced velocity Tactical loads, and, soft nose lead bullets up to heavy bullet in a 77 grain for those longer range or precision needs.
I've had a civilian version of the CAR-15 in 5.56mm for decades, and several years ago, upgraded to a couple Rock River gas tube M-4's that function well. So part of my decision to stick to the 5.56mm is financial - I already have several, and after 33 years of using, training on and teaching the AR variants, I guess I am just used to it.
I recommend the 4,000 rds per battle rifle as a Basic Load, as after a collapse, ammunition may just not be available. A serious firefight may take up several hundreds of rounds up to a thousand. Would not like to find myself low on ammunition after one or two serious dust ups. Avoiding fights when I can, but surviving the one's that are unavoidable. Would be great to have 10,000 rounds per main gun.....I am not get there I would surmise that most people simply are not that dedicated to Survival Preparation, nor financially able if they wanted to or have the financial will power to spend money on and stock these amounts.
Note: A Cursory search of 5.56x45mm ammunition prices show that 1,000 rounds of 55 grain FMJ (M193) to be around $460; while 1,000 rounds of 62 grain SS109 steel core penetrator (M855) to cost around $500.
Plus you should account for ammunition needed for training. If you bring in new people to your survival group, making them proficient or at least usable on your main rifles would be a good idea.
