*sighs*
It depends. We don't have enough info yet.
From personal experience, if it was a normal AR15 with a normal sighting system shooting normal ammo by a guy with a normal amount of training with the above firearm, yes, it's entirely possible to miss a head sized target from that distance. AR15s are not inherently accurate. They CAN be, but, many are not without a bunch of work, selecting the correct ammo, training to shoot it well, and then also selecting the correct time to take a shot.