ammunition, and the M103B1 is used with the M51A1B1/M51A2 dummy cartridges. Propellant All M50-configured cartridges are loaded with (double-base) ball spherical propellant. M52A3B1 Electric Primer The electric primer is used in all current M50 configurations. It consists of an open-ended brass cup that contains a brass button insulated from the cup by a plastic liner. The firing pin of the gun contacts this button. The ignition charge (a conductive explosive mixture) is in contact with the other side of the button, and is retained by a paper disk and a metal support cup. The electrically initiated primer ignites the propellant charge. NOTE: The primer explosive element is sensitive to electromagnetic and electrostatic energy. Projectiles All projectiles, except the HPT, have essentially the same external configuration. The rotating band is a copper alloy swaged into a circumferential groove near the aft end of the steel body. Fuze M505A3 Point-Detonating (PD) The PD fuze (fig. 7-3) consists of a body assembly, a rotor assembly, and a booster assembly. The fuze has a delayed arming distance of 20 to 35 feet after it leaves the muzzle of the gun. Before firing the HEI projectile, the rotor and the firing pin are locked in position by the rotor safety spring. (The rotor contains the detonator, which is out of line with the firing pin.) Centrifugal force causes the spring to open, allowing the rotor to move in-line with the firing pin. The fuze functions when the nose of the fuze is crushed against the target, forcing the firing pin against the detonator. The detonator, in turn, initiates the booster. The booster detonates and initiates the projectile’s explosive charge. Tracer A tracer mix is direct-loaded into a cavity machined in the base of the TP-T and HEI-T projectiles. It is used in assembling the M220 and M242 cartridges, Figure 7-3.—M505A3 point-detonating fuze (unarmed). respectively. The tracer is ignited by the heat and pressure of the propelling charge. The tracer is visible for about 1,280 yards of projectile flight. M51A1B1/M51A2 DUMMY CARTRIDGES Dummy cartridges are completely inert assemblies. They are used for drill and testing the feeder assembly of a weapon. The dummy cartridge is assembled with the M51A1B1/M51A2 projectile and M103B1 cartridge case to simulate the service cartridge. The two cartridges are the same except for a change to the annulus/primer pocket. The cartridge contains approximately 635 grains of inert material. This material produces an average overall weight equal to that of the other M50-configured ammunition. M254 DUMMY CARTRIDGE The M254 cartridge is made of plastic. It is an alternate to the M51A1B1/M51A2 steel dummy cartridge. You can tell the difference between the M254 and live ammunition by its appearance and by the feel of its nylon composition. M50 configuration characteristics are shown in table 7-1. 7-2