Overhead fire with the rifle is safe when the ground affords protection to friendly troops or when the troops are far enough below the line of fire (fig. 3-66). Whether or not overhead fire should be used in any particular case is a matter of judgment.

A machine gun on a tripod is capable of delivering accurate overhead fire because of the small, uniform dispersion of the cone of fire. In the attack, the use of overhead fire permits the machine gun to support the advance of the rifle units. To permit overhead fire on any target, pass the cone of fire over the heads of friendly troops by a certain distance, called the MINIMUM CLEARANCE. Two rules are prescribed by which the minimum clearance may be determined: the GUNNER'S RULE for ranges up to 900 yards and the LEADER'S RULE for ranges greater than 900 yards. Overhead fire will not be delivered by use of these rules at ranges less than 400 yards or greater than 1,800 yards.

The steps in using the GUNNER'S RULE are as follows:

1. Lay the gun on the target with the correct sight setting to hit the target.

2. Without disturbing the lay of the gun, set the rear sight at 1,500 yards.

3. Look through the sights and note the point where this new line of aim strikes the ground. If this point is beyond the feet of friendly troops, you may deliver overhead fire safely until the troops reach this point. It is not safe to fire when they pass this point.

The steps in the application of the LEADER'S RULE are as follows:

1. Select a point on the ground to which you believe friendly troops can advance with safety.

2. Determine the range to this point by the most accurate means available.

3. Lay the gun on the target with the correct sight setting to hit the target.

4. Set the rear sight at 1,500 yards or at the estimated range to the point plus 600 yards, whichever is greater, without disturbing the lay of the gun. Under no condition should the sight setting be less than 1,500 yards.

5. Note the point where the new line of aim strikes the ground. If it strikes at the selected point, that point marks the limit of safety.

6. If the new line of aim strikes the ground short of the selected point, troops can advance safely to the point where the line of aim strikes the ground and to an unknown point beyond. If you desire to fire after friendly troops advance farther than the point where the line of aim strikes the ground, you must determine this farther point by testing new points until the line of aim and the selected point coincide.