I call BS on this story. According to US Army ballistic tables for the M855 round, it drops 339.2 inches at 800 meters. The ballistics table doesn't go past 800 meters. A mile is around 1500 meters, and you would have better than 800 inches of drop at that range as velocity falls off quickly out there. No chance in hell of hitting, even if he pops off a full mag. Max effective range is 800 meters on an area target and 550 meters on a point target.
Not to mention that if his weapon was equipped with a Gen II, PVS-4 night sight, which is standard issue, the max range that he can see is 400 yards in starlight and 600 yards in moonlight.
Thanks for going to the trouble of listing the ballistic table for the round used, as well as max for visual ID.
And you did not even mention the kinetic energy of the bullet at that extreme range. The bullet probably would have if it hit his head just bounced off and left a bruise. As for the scope's maximums, again roger that.