I've seen very little in the way of parts failure on AR's and (in the NG) the M-16A1. I have seen loss of small parts, especially springs, more often. So, I'd give you two pieces of advice:
1. [b]Don't strip the weapon past the basic field strip[/b] unless you have a good and pressing reason to do so. If you do, do it indoors and set things up so that if any little springs go sailing out of your grasp they will land where you can find them.
2. Buy one of the small parts survival kits from Bushmaster or the other major makers. Buying a whole separate bolt and carrier works too, but the point of doing that is mainly to be able to drop a new bolt in if there is extractor failure or drop a new carrier in if there is a problem with the gas key instead of replacing the small part itself that failed. An added virtue of buying the whole bolt system, I guess, is that you are back up and firing in a couple of minutes w/o the necessity of tools. You can then repair the broken one at leisure.