3-4000 rounds? I have to differ there Luis. My brand new AGM bcg started to crackle after ~200 rounds, so had to buy a new one(put an plastic NPAS into it too), and my friends gun had the following problems: after ~1000 rounds: trigger sear broke, ~2000 rounds: bcg cracked into two. Both of us use green gas.
So I would recommend changing the following to make the gun somewhat durable: (I try to stay at the low budget side) Anti rotation links,an aluminum bolt carrier, plastic NPAS, steel boltcatch, and a chinese PGC Hop chamber copy. And after this, changing the parts as they broke.