They were repeatedly shooting vehicles moving 500 miles per hour from the top of a cliff! You don't get any more precise than that. 
Though I will say that kids watching ANH for the first time today probably crap their pants when the tusken raider aims his rifle at the tiny speck that is Luke's sandspeeder.

Though I will say that kids watching ANH for the first time today probably crap their pants when the tusken raider aims his rifle at the tiny speck that is Luke's sandspeeder.