People should buy extra spare parts that can fail like firing pins. what others? Springs?
Springs, seals, trigger parts, Firing pins, Takedown Pin, two bolts, large capacity magazines. I would suggest a barrel or two.
I have 10K rounds of .223 Rem (5.56x 45), 2000 rounds of .308 (7.62 x 54) and several thousand rounds of 00 shot.
That's very good advice. I would add sears, extractors and magazine springs. Gun-specific tools are important, and spare optics wouldn't hurt.