Ammunition for the 4.2-inch mortar was of the semifixed complete type. All parts of the round were loaded in the mortar as a unit and in one operation, but the propelling charge was varied for different ranges. A complete round consisted of a fixed shell, fuze, and a propelling charge. The propelling charge consisted of an ignition cartridge and a number of powder charges. The ignition cartridge was inserted into a cartridge container that was screwed to the base of the shell. The powder charges were placed on the outside of the cartridge container and held in place by an adjustable propellant holder and a striker nut.

When fired, the shell was stabilized in flight by rotation transmitted to the shell by means of the pressure plate expanding the rotating disk on the base of the shell thus forcing the disk to engage the rifling in the bore. The shell, which had a deep cavity and suplementary charge, was fitted with a point detonating fuze. The ignition cartridge was housed in the cartridge container extension and was held in place by the striker nut which contained the striker. The propelling charge consisted of a number of increments of propellent powder in the form of square sheets assembled on the cartridge container. When the round was inserted into the bore and released, it slid to the bottom where the firing pin drove the striker into the primer of the ignition cartridge. Flame from the ignition cartridge flashed through vents in the cartridge container extension to ignite the propellant, thus firing the round.

Because of its size and weight, the weapon was used as Regimental artillery, often vehicle mounted. A 4.2 mortar squad had 8 men, with 4 squads in a 4.2 in. mortar platoon. In the Marine Corps, each Rifle Battalion had a Weapons Company, with 60 and 81 mm mortars, and heavy machine guns. Each Rifle Regiment had a 4.2 inch mortar company with 12 guns, supporting the Rifle Battalions as needed. These heavy mortars were almost as effective as the Chinese 120 mm, the heaviest artillery available to the infantry divisions when they assaulted Eighth Army and X Corps at Chosin.

Song Shilun's 9th Army Group, consisting of 4 armies, of 12 divisions and about 120,000 men, threw almost all its strength at the three Infantry Regiments of the US First Marine Division. The successful withdrawal by the Marines, while destroying the CCF divisions as effective fighting units for the balance of the Korean War, was greatly assisted by the fortunate proximity of Divisional light and medium artillery, and constant and devastating air to ground support. But that was not the main reason 1st Marine Divison survived. Eighth Army on the West Coast had even more artillery and air support, and much more armor, and yet they were completely routed by a CCF force approximately of the size that was fought off by the single division of Marines.

The basic reason the Marines performed so brilliantly was that they were psychologically prepared to fight, and to die if necessary, and were thoroughly trained and experienced in effectively deploying all the basic weapons of an Infantry Division. In that regard, the 60 mm mortars deployed with each platoon, supported by the 81 mm mortars at Battalion, and even more powerfully by the 4.2 in mortars at Regiment, were well targeted, and savagely effective.

It was the men who closed with the CCF at the unit level, fought them to a standstill, identified appropriate targets and exploited the use of mortars and machine guns that were deployed against those targets, who made the difference.

In war, it will always be the men who make the most vital difference. But, it sure helps for them to have plenty of effective weapons at hand!