It would be overkill, but how about an extra LPK, an extra BCG, an extra buffer and buffer spring?
Or just get some extra extractor kits and firing pins.
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Gas rings $3, extractor spring $5, action spring $5.
Buy at least one extra spring of each type in the AR. Get at least an extra extractor & pin, firing pin & retaining pin, gas rings as mentioned above a couple complete bolts and lower parts kits if possible. Some of the items you may lose like pins and the others will wear and/or break with use.