We used 36 yard BZO in the corps. Here is the explanation from a USMC Weapons Training Battalion
"When a rifle is zeroed at 300 yards, the bullet will cross the line of sight twice. The bullet will cross the line of sight first on its upward path of the trajectory at 36 yards, and again farther down range at 300 yards (point of aim/point of impact). When the bullet’s trajectory intersects the line of sight at 36 yards and 300 yards, the rifle is considered to be zeroed to hit a target at the exact point of aim at both of these ranges. That is why there is an alternate, field expedient method for zeroing the rifle at 36 yards when a 300-yard range is not available. "