The H&K416 w/piston looks to be a better varient than the Colt:
Discovery Channel testing:
YouTube - Heckler & Koch - HK416
History Channel testing:
YouTube - The awesomeness that is the Heckler and Koch 416
Another:
YouTube - Future Weapons Season 2 Episode 6(First Strike): The HK 416
There's a video where a guy runs clip after clip after clip through it on full auto, then switches to 100rd drums and the thing just keeps going.