An SBR.
Seriously. Every time I have too many "spare parts" they magically become a complete gun and I no longer have spare parts. It's happened three times already! Learned my lesson... no more spare parts for me.
When you say "spare parts kit", are you talking portable, often carried, or "in stock at home"?
Ah... Such a simplistic life view. There are so many conditions that could invoke stress. Failures tend to happen at inopportune times. That's why people carry parts with them and not just at home. How about gun breaks or even losing a spring during a competition or a training class?What “stress”? Gun breaks, stop shooting, fix it.
Or go home and fix it.
No one asking about spare parts on a forum is stuck in a jungle at night under fire with a broken AR.