So I've got this AR, its a 20" heavy barrel with standard A2 iron sights, never fired and freshly assembled.. I recall that there was a way that you could sight it in at a short distance, like 15yds or something, where when sighted in at that distance you would be on target at 100yds. Then you should be equally on target at 200, 300, etc by adjusting the rear sight accordingly... Anyone know what that distance is? Or direct me somewhere?