I often wonder where people plan on buying repair parts after a SHTF situation. In a true situation of that magnitude, you won't be able to buy much of anything. What you have is what you use. If you think that your weapon will need spare parts, better buy them BEFORE. Sort of negates the "parts being hard to find" objection.
1. AR and AK parts are cheap and easy to get NOW, which makes it easy to have two or three extractors, firing pins, etc. I believe it is neccessary to deal directly with Ruger to get any parts now and some parts, such as a bolt, require you to ship the rifle to them to make the repair. You can walk into a gunshop, or pick up a catalog, and get whatever AR or AK parts you need.
2. It is quite possible that if and when there are no places available to buy parts, an AR, AK or M16 will be around; don't you think? Odds are, the "good guys" or "bad guys" will be using one or the other.
Just my two cents.
Magazines would likewise probably be anywhere that individuals were using AR/M16 or AK rifles, right?