Which parts on the AR15 are most common to have issues or breakages that will render your weapon useless? Here is a list of parts most likely to break that you should keep on hand. Extractor spring insert, Extractor spring, Cam pin, Trigger Spring, Firing pin, 2 Firing pin retaining pins, Safety detent, Extractor pin, Detent Spring, 3 Gas Rings, Disconnector, Hammer Spring. These should go wherever your rifle goes. You can buy field repair kits for about $30.
These spare parts should make sure that you are not out of the fight for long if something minor should break. I also like to have a spare bolt on hand. Of course the very best spare is another complete rifle, however, although I own several AR-15’s, I also have spare parts on hand as well. Just in case.