I recall a specific instance where my Nephew and I were shooting an Argentine Mauser at a distance of roughly 440 yards.
It was not hard at all to hit a tar bucket (I know, it's larger than a head). My Nephew is not a particularly good shot and he was hitting it fairly often too. The bucket made a good target as it was for some reason still full of tar and would move with each hit.
The fact that the rifle was zeroed by a German in 1909 for that same distance didn't hurt (a guy named Herr Ritzmann had signed the original test target which came with the rifle. He had shot a two inch group at 200 meters and the shots were about 4 inches above point of aim). I would hate to have someone who was a good marksman shooting at me even at longer distances and with open sights.