The guy you responded to has no idea about guns. You did a lot better, although I would note that an accurized M14, the target shooting brother of your service grade rifle, can easily hold headshots at 500 m. Snipers use actorized semi Autos also. You can do wonders with an AR rifle but it surely wasn't on a 10-year-old DPMS model as was used here. DPMS was always notorious for crappy barrels. Then you need to wonder whether the kid was shooting the right ammo through the rifle as there are different twist rates for different bullets.