The gunner can only have one of two sight pictures when the mortar is fired: either the ALIGNED sight picture or the COMPENSATED sight picture. To use the aligned sight picture correctly, the gunner must lay the vertical line of the sight reticle, so it just touches the left side of the visible aiming post (fig. 14-16).

To use the compensated sight picture correctly, the gunner must lay the vertical line of the sight reticle, so the left edge of the far aiming post is placed exactly midway between the left edge of the near aiming post and the vertical line of the sight reticle. This corrects for displacement of the sight (fig. 14-17).

SIGHTING

The gunner is responsible for setting the mortar to the correct deflection and elevation. Remember, when the gunner lays the mortar, the sight is first set for deflection then for elevation, while the mortar is first laid for elevation then deflection. You can readily remember this sequence by keeping in mind the key word DEED.

1. Place Deflection on sight

2. Place Elevation on sight

3. Level for Elevation

4. Level for Deflection

For example, we will use the fire command for an HE round at a deflection of 2,700 mils, charge two, and an elevation of 1,150 mils. Assume the mortar is facing the principal direction of fire and the sight is aligned with the aiming posts at a deflection of 2,800 mils.

Setting the Sight

The gunner first sets the new deflection of 2,700 mils on the sight unit. To do this, the gunner rotates the deflection knob counterclockwise until the index points to 2,700 on the deflection scale and to 0 on the micrometer scale. Notice that moving the knob counterclockwise has moved the sight to the left. A clockwise movement would move the sight to the right. The gunner should always keep in mind that the barrel moves in the opposite direction of the sight. In addition, the gunner must know in which direction the barrel is pointing after a given deflection change. Any deflection command that causes the barrel to be moved over 90 degrees either right or left of the principal direction of fire (PDF) is always given as a REAR deflection. (For example: DEFLECTION REAR TWO-SEVEN-HUNDRED.) Notice that a rear deflection is directly opposite the equivalent FRONT deflection. Any deflection without the command REAR is understood to be FRONT.

After rotating the sight to the new deflection of 2,700 mils, the gunner sets the new elevation by rotating the elevation knob until the index on the elevation scale points between 1,100 and 1,200, and the index of the elevation micrometer is pointed at 50 nils.

Laying for Direction

As the deflection placed on the sight is greater than 95 mils, it cannot be made by turning the traversing crank. Therefore, the gunner assisted by the assistant gunner must lay the barrel in the new direction of fire by moving the biped. Whenever the biped is moved in this manner, the gunner should always rotate the barrel to the center of the mechanism to give maximum flexibility at the new setting. As a rule of thumb, the biped should always be moved whenever the new deflections more than 20 mils either side of center. Care should be taken in moving the biped to get roughly within two turns (20 mils) of the aiming posts to avoid excessive traversing.

With the barrel in its new position and the vertical cross hair aligned with the left edge of the aiming posts again, the gunner next levels the barrel for elevation by turning the elevating crank until the bubble of the elevation level is centered. The gunner then turns the adjusting nut of the cross-leveling assembly until the cross-level bubble is centered. After a final check with the aiming posts, the mortar is now laid in the new direction of fire and is ready for firing.