Military

a. Upon receiving the call for fire, the section sergeant determines from the size and description of the target that traversing fire must be used to cover the target. (To effectively engage a target using traversing fire, the section sergeant ensures the attitude of the target is within 100 mils of the attitude of the firing section.) The section sergeant then completes the FDC order (Figure 13-1).

b. The three or four mortars are plotted separately on the M16/M19 plotting board, using the attitude of the section. During the mission, the computer ensures that the correct plots are used to determine data required--for example, during the adjustment, the impact point is aligned with the No. 2 mortar plot. Using the information in the call for fire, the FDC order, and the observer corrections, the computer computes the data to adjust the No. 2 mortar onto the center mass of the target. After the adjustment is complete (Figure 13-2), the computer must complete the following procedure:

• Plot the 250-meter length of target on plotting board using the attitude of the target.
• Divide the target into segments.
• Determine the number of rounds for each segment.
• Determine the mil width of one segment.
• Determine the number of turns required to cover one segment.
• Determine the number of turns between rounds.

c. To plot the target on the plotting board, the computer rotates the azimuth disc until the target attitude (taken from the call for fire) is indexed. The computer erases all the plots except the last plot. After ensuring that the attitude is indexed, the computer divides the total target area into segments. These plots represent the starting points for each mortar. The area between the plots is the area each mortar must cover with fire (Figure 13-3).

d. The target is now divided into three segments. Once the remaining data for one segment have been determined, the data will apply to all three mortars. Since each segment of the target is 75 meters, if the computer determines the mil width of one segment, the other two will be the same. The computer can use one of two methods to determine the number of mils for one segment.

e. To compute the number of turns to take between rounds, the computer must know how many rounds will be fired for each segment. This information is given in the FDC order (3 rounds). To determine the turns between rounds, the computer divides the total turns by the interval between rounds (there will always be one less interval than the number of rounds: 3 rounds = 2 intervals).

f. At this point, the computer must determine the deflection and range for each mortar by aligning each mortar with its start point, completing the subsequent command, and issuing it to the mortar section. If there is a range change of 25 meters or more, the mortar will receive its own elevation.

g. Upon completion of the adjustment phase of the mission, the section is given the command PREPARE TO TRAVERSE RIGHT (LEFT). The gunners then traverse the mortars all the way in the direction opposite to that given, back off two turns, and await further instructions (Figure 13-5).

a. Searching Fire. For the mortar section to effectively engage a target using only searching fire, the attitude of the target cannot be more than 100 mils difference from the attitude of the gun section. If the difference is more than 100 mils, the target should be engaged using a combination of searching and traversing fire, or traversing fire only. When the section is firing a searching mission, the adjustment phase of the mission is the same as a regular mission using the base mortar (No. 2) as the adjusting mortar. The base mortar is adjusted to center mass of the target.

b. Zone Fire. The 4.2-inch mortar does not fire a search mission the same as the 120-mm, 81-mm, 60-mm mortars. It does not have the same elevating characteristics as the other mortars; therefore, the 4.2-inch mortar uses zone fire when targets have more depth than a platoon/section can cover when firing a standard sheaf. The 4.2-inch mortar platoon/section usually fires two standard zones: a 100-meter zone (three rounds for each mortar) for a platoon-size target, and a 200-meter zone (five rounds for each mortar) for a company-size target.

a. Observers. Observers who are to adjust illumination should be informed when the 81-mm mortars are firing M301A3 illumination rounds. The M301A3 has an HOB of 600 meters, while the M301A1 and M310A2 rounds have 400-meter HOBs. There is a difference in adjustment procedure. The M301A1 and M301A2 rounds are adjusted to a ground-level burnout; the M301A3 round should have a burnout 150 to 200 meters above ground. This procedure is based on the fact that all three of the rounds fall at a rate of 6 mps (Table 13-2).

b. Corrections. The ranges in the firing tables are in 50-meter increments. (Rule: Always round up, such as range 2,525 meters = 2,550 meters, to enter Part II of the firing tables.) Corrections to the HOB are obtained in columns 4 and 5. These corrections are used to move the round up or down in relation to the HOB line (Figure 13-10 and Figure 13-11).

c. Adjustments. The round is fired and the FO sends: ADD TWO ZERO ZERO (200), UP ONE ZERO ZERO (100). The computed range is now 2,725 = 2,750 (Figure 13-12). The basic data only give an HOB of 600 meters, but the FO requested an UP 100, meaning that the round needs more height. To compute this change, the computer must determine where this round will be in relation to the HOB line: HOB = 600 meters; UP 100 is two increments above the HOB line. Once the number of increments has been determined, the computer goes to column 4 (change in elevation for 50-meter increase in HOB) and column 5 (changes in fuze setting for 50-meter increase in HOB), and multiplies the increments times the correction factors given in these columns.