A spare bolt assembly will likely fix any breakge you encounter. Other than that, I like to keep a couple spare firing pins, cam pins, firing pin retainers, gas ring sets, and extractor springs and inserts. An extractor, carrier key and screws, buffer spring, and lower parts kit is not a bad idea either. It really depends on what level of preparedness you are looking for. Bushmaster makes a nice spare parts kit for around $40 or $50, it includes at least one of every spring, pin, and detent in the gun.