What was the practical purpose of the Sunderland's four fixed forward machine guns?

As a countermeasure to the increasing defensive armament of the U-boats, the Australians field-fitted their aircraft with four additional fixed .303s (7.7 mm) in the nose, which let the pilot add fire while diving on the submarine before releasing the bombs. Most aircraft received a similar modification.

The nose guns were later reinforced with four more fixed machine guns, two on each side, in the forward fuselage, fired by the pilot. Throughout the Mk III's service life a large number of almost continuous improvements were introduced, including the ASV Mk IIIA and four more fixed machine guns in the forward fuselage wall, just behind the turret, first developed on RAAF aircraft and with a simple bead-and-ring sight for the pilot.

The nose turret used two different armament fits over time; the more common one, later on, was two Browning machine guns. The defensive armament included a Nash & Thompson FN-13 power-operated turret with four British .303 Brownings at the tail end and a pair of manually operated .303s on each side of the fuselage, firing through openings just below and behind the wings. Much later a two-gun dorsal turret would be fitted, on top of the fuselage roughly level with the wing's trailing edge, raising the total defensive armament to 16 machine guns.