M4 Mortar Sight
Transcribed - 1/1/2013
From the Infantry Journal, 1942:
The Sight M4 , provided with the 81mm mortar for laying in elevation and direction is likewise the standard sight for the 60mm mortar. The sight includes a collimator, elevating and lateral deflection mechamishms, and longitudinal and cross levels, asll supported by a bracket with a dovetailed base which fits in a slot in the mortar yoke and latches in place.
The levels when centered indicate the elevation and eflection angles, respectively, to be mreasured in true vertical and horizontal planes. The longitudinal level also provides a datum line for elevation settings. The collimator, the direction sighting device of the sight, consists of a vertical line located at the focus of an eye piece, all enclosed within a rectangular tube. When the sight is level, the collimator establishes a vertical line in the field of view, the line of sight with a normal lateral deflection setting being parallel to the plane of fire. The collimator is pivoted so that it may be moved in elevation as required to bring the aiming point into the field of view; this motion has no effect on elevation indications. The sight is removed for traveling, and a carrying case for it is provided.
The Sight M-4 is an accurate, ruggedly constructed instrument used to lay the mortar for direction and elevation.
It provides the means for mounting the sight, for laying in direction and elevation, for cross-levling the mount, and for determinig mask clearance.
Elevation in degrees is indicated on the associated scale, graduated in 10° steps, supplimented by a micrometer graduated in 1/4° steps. Deflection in mils is indicated on the associated micrometer which is graduated in -mil steps. Directions of motion for left and right deflections are indicated by the letters L and R and arrows near the index. Deflection is limited to ~ 150 mils and a zero indication corresponds to the normal setting (line of sight parallel to the line of fire).
The collimator and the open sight directly below it have vertical reference lines and may be placed as desired in elevation. With the collimator moved to the extreme rear and an elevation setting of 40°, the elevation of the overhead portion of th eopen sight is 2°below the axis of the mortar tube, a feature of service in determining the approximate minimum elevation for clearing nearby objects.
