Hathcock did not use a custom anything to hold up the M2, he just dialed in the T&E to make the shot. That was a daytime exercise sighting in where Charlie was known to refill his canteen on a river bank. Actually a no brainer with the issue M2 tripod, it's that good.
Exactly. This is why taking an old Mosin Nagant and claiming because it will shoot within 2.5" at 100 yards, it will shoot within 20" at 800 yards is wishful thinking. Group sizes tend to expand as distance increases, not only because of shooter and environment related issues, but also because of bullet issues. MOA does EXACTLY that, explains things in Minutes of Angle, which will increase at longer ranges, creating a larger group size precisely because it's describing a cone of dispersion.
Aside from the anomaly of "sleeping bullets," most firearms will always have an increasingly larger group size as distance increases. A firearm posting a 1.5 MOA group at 100 yard cannot possibly shoot 800 and get a smaller group. If the dispersion gives you flyers out at the inch and a half mark, it will be 3 inches at 200, 6 inches at 400, etc. Bullets cannot magically return to the center of a group just because they flew further and somehow lost dynamic wobble. Consider just the lower shots in the group, and explain how a bullet traveling under the center can rise and meet the target point 800 yards further, when the high shots wouldn't rise and move the shot group higher.
I think we need to leave "sleeping bullets" lie and just treat MOA for what it is, a gradually increasing cone of dispersion as distance increases. If your gun is 1MOA at 100 yards, that's a 1" group, and a 10" group at 1000 - which could be done in a Texas warehouse on weekends.
Outdoors, good luck with that, it's the whole point of the challenge. Wind, mirage, and other factors may influence shots differently, but fire a burst of rounds and the cone of dispersion will be affected largely by the same influences and fall in the MOA. As any MG gunner knows.
It's those pesky single shots that suffer variables more, walk a belt of ammo in and no problem. Then, the MOA ensures coverage and you will get a hit. Takes a lot less time, too.
Just some of that professional instruction I was paid to learn, using ballistics as an effective tool, not an academic debate.