Diamantina’s main armament was 2 single 4” (102mm) Mark XVI guns on Mark XX high angle/low angle mountings, one forward and one aft. They could be used for surface gunfire or against aircraft. The mounts were manually trained and elevated and the guns manually loaded. Aiming was by optical sights mounted on the gun mount. The weight of the gun was 2 tonnes (2 tons) and the total weight of the complete gun mounting was about 1.2 tonnes (10 tons).

The gun was of Quick Fire design. That is, the breech opened automatically after each shot and the spent casing (the brass cylinder) was ejected. The gun was then ready to receive the next round.

On both outsides of the gun shield were metal rails used for launching rocket flares.

As there was no fire director on the River Class frigates, aiming calculations were carried out by the Gunnery Officer on the bridge. The resultant range and deflection data was relayed to the gun mounting by voice pipe and the aiming telescopes adjusted accordingly. The gun was brought to bear on the target using the aiming telescopes, one each for both the layer and the trainer.

A usual gun crew consisted of 8 men, the gun captain and breech worker, the gun layer on the left side (elevation), the gun trainer on the right side (left/right movement), 3 loaders to carry the rounds and load the gun, a communications number (to receive and relay orders from the Gunnery Officer) and the fuse-setter who was positioned in the gun shelter. Although each man had his role to play in the team, each was expected to be able to take over any other role. A good gun crew could fire off 10 to 12 rounds per minute.

The primary ammunition carried in the River Class was high explosive. As the ships were not expected to be in a ship to ship engagement, no armour piercing rounds were carried. Rounds could be fused with contact fuses for surface engagements or with time fuses for anti-aircraft barrage. Small quantities of star shell were included, as well as some practice rounds.

Both guns had access to 2 ready-use lockers located outside each gun shelter. Each locker contained 32 rounds. One locker held rounds with contact fuses while the other held rounds with timed fuses. An ammunition hoist extended from inside the gun shelter down to the magazine which was 4 (forward) or 3 (aft) decks below. The forward magazine held 204 rounds of 4“ ammunition and 52 rounds of star shell while the aft magazine held 186 rounds of 4” ammunition and 48 rounds of star shell.

A complete round weighed 28.8 kg (63.5 pound), with a shell (projectile) weight of 15.9 kg (35 pound). Range was quoted as 18 150 m (19 850 yards). Rounds were fitted with the required fuse screwed into the nose of the shell. The timed fuse was set by the fuse setter in the gun shelter before being loaded into the gun.