57mm Gun Motor Carriage T48.
The 57mm GMC T48 was a very similar design to the 75mm GMC M3. This vehicle, the pilot T48, is armed with the short barrel 6 pdr Mark III. Production vehicles were fitted with the American version of the 6 pdr Mark V, which had a longer and thinner barrel than the weapon featured here. This half-track has the fender-mounted headlights, the armored windshield is folded down, and the box on the running board housed batteries. The ruler in the photograph is 73" (185cm) tall. (Picture from Standard Nomenclature List G-102.)
This view allows us to see into the fighting compartment of the pilot T48. Fuel tanks were placed at the rear corners of the vehicle, and the 57mm ammunition rack, visible with the two bags on its top, was between these. (Picture from Tank Data, vol. 2.)
The reticle for the telescope M18 is drawn above, but note that the parenthetical notations in the sketch did not appear in the actual reticle. The M18 was a 1x straight telescope, and the reticle represented 1½° deflection to the right and left, graduated at ½° intervals. The "R" and "L" etched on the reticle helped the gunner aim at moving targets: if the target was moving to the right, the lead markings over the "R" were used, and vice-versa. The telescope mounts M24 and M24A1 used with the M18 had independent elevation mechanisms that were graduated to allow the gunner to lay for initial estimated range. With the M24, the 300-yard (270m) setting was used from 0-300 yards (0-270m), the 700-yard (640m) setting for ranges from 400-700 yards (360-640m), and the 900-yard (820m) setting for ranges over 700 yards (640m). With the M24A1, the 500-yard (460m) setting was used for targets from 0-500 yards (0-460m), the 700-yard (640m) setting up to 700 yards (640m), the 900-yard (820m) setting up to 900 yards (820m), the 1,100-yard (1,000m) setting up to 1,100 yards (1,000m), and the 1,300-yard (1,200m) setting up to 1,300 yards (1,200m). Beyond this, the scale was graduated in 100-yard (91m) increments up to 2,500 yards (2,300m). Targets between these ranges required interpolation by the gunner, and to assist with this, changing of aim from the horizontal crosshair to the end of one of the vertical lines labeled B through F in the image would change the range by ~400 yards (~370m). (Picture from FM 23-75 57-mm Gun M1.)
