Cartridge, 105 mm: HERA, M548

Cartridge, 105 mm HERA, M913 (which see). The 105 mm Cartridge HERA, M548 is a semi-fixed round with the projectile being a free fit in the cartridge case to allow free access to the propellant increments inside the case. The projectile is a hollow steel forging with a rocket motor assembly attached to the base. The body is filled with approximately 2.36 kg of Composition B, deep drilled beneath the fuze well to accommodate a 136 g supplementary charge of TNT pressed into an aluminium sleeve. The rocket motor assembly consists of a body with a conventional boat-tailed outline with a central extension (spike) protruding from the base. The rocket motor body has a sintered iron driving band around its upper part next to the junction where it joins the shell body, and the motor itself contains 454 g of rocket grain propellant. The base extension houses an ignition delay assembly loaded with a pyrotechnic mixture and fitted with a rocket selector cap. The 105 mm Cartridge HERA, M548 has an intermediate and an extended range capability. For intermediate ranges the M548 is fired using Charges 3 to 7 with the rocket selector cap in place. For extended ranges the projectile is fired using Charge 7 only and with the rocket selector cap removed. On firing, the propellant gases ignite the pyrotechnic delay mixture. After a fixed delay of 16 seconds the mixture ignites the rocket motor which cuts in for two seconds at a critical instant during the projectile's trajectory, using its thrust to augment the projectile velocity and thus extending the range. On impact the M548 functions in the same manner as the 105 mm HE M1, but is claimed to be significantly more effective than the older projectile. The cartridge case may be either the M14 , M14B1 or M14B4 type fitted with an M108 percussion primer.