I've never been in a combat situation, so these are just my uninformed thoughts. I will happily stand corrected by any combat veteran.
Assuming you are in a defensive position only, with cover, I would tend to think that a bolt action rifle with a 4x scope is going to be every bit as effective, if not more, than an AR or AK. History is full of examples of snipers, in both Europe and the Pacific Islands, pinning down entire platoons and even companies of American troops and significantly depleting their numbers for days on end.
The ability to hit a target at short, medium and long distances with a single shot is a huge advantage, not a disadvantage. Spraying 20 round magazines of .223 down field, which is exactly what everyone would end up doing in a high stress situation, will not result in more hits, will use up ammo at an unacceptable rate, and will show all 20 guys in a platoon your exact location.
I wonder how many thousand rounds of .223 were fired by our troops using ARs in the jungles of Vietnam for every enemy soldier actually killed by it? Compare this to the question of how many rounds of .303 British ammo in an Enfield, or .30-06 ammo in a Springfield, were fired for every enemy in WWII that were killed by it.
From what I have read, it was not uncommon in Vietnam, for VC with old Moison Nagant full length rifles, to effective engage our platoons of our troops in the jungles and patties who were armed with the AR.
Different terrain? The Afghans, using 50 year old Enfields, had no trouble effectively engaging crack Russian troops armed with AKs in the open mountain country.
I hope I haven't offended any Vets, and would welcome their input.
I notice that most folks who toute the AR, and 20 round mags, also routinely talk about "holding off" folks, and "re-supply." In a SHTF scenario, holding off would not be good enough, and their would not be any re-supply. I believe that what is needed is a sufficient number of one shot hits, until the point is reached that the attackers decide you are more trouble that your supplies are worth. A 20 to 25% casualty rate is generally enough to stop any group of aggressors in combat. After that, their "structure" seems to break down.
Mannyrock