....you may say to yourself "Only an idiot would do that! I'll never do that....".....and for a while, you may not......Firing pin, buffer.
If your gas is set right the buffer will last a long time.
Possibly a gas adjustment valve some have launched them and never found it.
Kermit (Grampz) used to put together a small parts kit, well worth the money
Yeah, I broke the proprietary FCG spring....think its the trigger spring IIRC. Gun still worked.....can't remember if it doubled or what though. Had a broken firing pin too....Type 1 gas plugs have been known to launch. Type 3 can't without a significant failure.
As mentioned, firing pin and buffer are a good choice. Extractors and springs, ejector and bolts/ washers, fcg springs. Don't remember any extractor or ejector issues but I've personally replaced a trigger spring. There have been some cracked carriers but both I know about were Type 1. Broken op rods but it's been awhile. Old bhos had some failures but I haven't heard about a Type 3 going yet.
I had some RA packaged spare parts kits pretty sure I'm out. I can go through my small parts boxes tomorrow and see what I've got on hand.