I know of no one who personally puts more rounds down the barrel of AR-15/M4 type weapons each year than Mike Pannone. I’ve had the honor of shooting with him several times, and came away from those brief sessions a better shooter. I greatly respect his opinion and experience on all things AR-15/M4.
He’s recently completed an exhaustive test of a Bravo Company 14.5″ M4 type upper receiver assembly. You can read the details here. In essence, he stripped away all lubrication, replaced critical items in the lower receiver, and proceeded to shoot 2400 rounds through the weapon with only magazine-related failures (he used USGI type mags to simulate as closely as possible the military M4).