How many machine guns did the Sunderland end up with, compared to the original six?

The initial defensive armament included a Nash & Thompson FN-13 powered turret with four British Browning .303s at the end of the tail and a pair of hand-operated .303s, one on each side of the fuselage, firing through openings just below and behind the wings. A second gun was added to the nose turret. On the Mark II, the tail turret became an FN.4A, which kept the four .303s of its predecessor but offered twice the ammunition capacity, with 1,000 rounds per gun.

On the Mk IIs the guns in the side hatches were removed, while the Mk IIIs and later the Mk Is received the far more capable .50s (12.7 mm), one on each side. It also became common to add a flexibly mounted .50-inch M2 Browning in each of the side hatches, behind and above the trailing edge of the wing. The Australians equipped their aircraft in the field with four additional .303s (7.7 mm) on fixed mounts in the nose, which allowed the pilot to add fire while diving on the submarine before releasing the bombs. Among the many almost continuous improvements introduced during the life of the Mk III were the ASV Mk IIIA and four more machine guns in a fixed position in the forward fuselage wall, just behind the turret, developed first on RAAF aircraft and fitted with a simple ball-and-ring sight for the pilot.

Much later a two-gun turret would be fitted in a dorsal position, on top of the fuselage and roughly level with the trailing edge of the wing, bringing the total defensive armament up to 16 machine guns.