AR 5.56 is wrong tool for a 400 ft shot? Really?Not to start a fight, but, I have been to a range or two, and just because folks are at a range and have a range finder doesn't make them knowledgeable.
The tool(s) of choice, position, etc say he was less than knowledgeable, at least in the task he was attempting.
I saw a pic, (Can't find it again), that looked like a RedDot on the AR he had on the roof. And, an AR 5.56 wasn't the right tool.