Avoid piston ARs. They're a shitty bandaid fix tacked on to solve an imaginary problem, and have problems of their own (carrier tilt, decreased accuracy, decreased parts life). They're actually less straightforward than a direct impingement AR as they add more parts. In a DI AR the gas acts on the BCG itself, effectively making the BCG a piston.
As for NEA I've heard of problems but not seen any NEA rifles firsthand.
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