Spare parts, extractors, springs, strikers, firing pins, ect. The parts most likely to wear out or break. I can have all the ammo and mags in the world but if the gun goes down over an inexpensive easily replaced part wearing out or breaking then it does me no good. Of course it doesn't mean I'm not also stocking up on the other goodies as well.
Do you buy "oops kits" for this or do you piece these parts together on experience from breakage? Spare bolts or BCG'S?