Here is an interesting article about one of the new Hybrid rifles where polymer is replacing aluminum for gun parts. The real remaining question is how long will it really last? Will it survive say 50,000 rounds? The article talks about testing for 2000 rounds which for many guns is a life time of shooting. But a real life cycle fatigue test would run at least ten times that many rounds through it . So time will tell but it is certainly promising.
take care and be safe