Let's say the kill zone in the head is 6" in diameter. At 500m, this subtends an angle of 7 minutes of arc. For comparison, the Moon is about 30. That's 1/4 the diamemter of the Moon. To do that consistently would require a very good marksman. But is is possible.
In contrast, a 6' man is 12 times as big, or 3 times the size of the Moon - or about the size of the Moon if you just consider the torso. I could hit that with a slingshot!
I suppose the M16 has a very high muzzle velocity so the slug flies true.
Lee Harvey Oswald and Charles Whitman (UT clock tower) were famous shooters. What did they have in common?