I am putting together a list and then a collection of spare parts for my AR 15. By this I mean parts which are most prone to failure. So far I have gas rings for sure. Trigger springs but I have a drop in trigger so this is not on my personal list. Firing pin retainer pin (cotter pin). Cam pin. This is one replacement item which is interesting because it takes so much abuse you would think it would fail more often, although mine never has. Firing pin--everyone says you need a spare. Extractor spring--donut?

What else have I missed? What has failed in your case?