Mask clearance is the clearance required for firing the weapon without danger of a premature detonation of the round by obstructions, such as trees, hills, or buildings. After the mortar is mounted, the gunner must determine the minimum and the maximum mask clearance. This determination helps to ensure troop safety at the mortar position and also helps the fire

Figure 14-17.-Compensated sight picture.

direction center (FDC) to know whether or not a particular mission can be completed with that mortar. To determine mask clearance, the gunner sets the sight at 0 deflection and 0 elevation, making the line of sight parallel with the axis of the bore. The gunner then raises or lowers the barrel until the cross hairs of the sight just touch the obstruction (tree, hill, etc.). The gunner then levels the elevation bubble by turning the elevation knob on the sight and reads the elevation from the elevation scale. This reading is the minimum (or maximum, depending on the location of the obstruction) mask clearance. For a safety factor, 30 mils is added to (or subtracted from) the elevation reading.

Mask clearance must be determined in a complete circle around the mortar to every object that could interfere with firing. The gunner reports all these figures, stating the azimuth and clearance to the squad leader and FDC. He also keeps a record for himself. In emergency situations, where time is extremely important, the gunner can quickly determine the mask clearance for any given target by placing his head near the baseplug and sighting along the top of the barrel. When the line of sight clears the mask, it is safe to fire. Of course, it takes considerable time to sight this way before every fire mission; therefore, at the first opportunity, determine the minimum and maximum clearance by using the sight and then record it.