QF 2-pounder naval gun
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The 2-pounder gun, officially the QF 2-pounder and universally known as the pom-pom, was a 40 mm British autocannon, used as an anti-aircraft gun by the Royal Navy. The name came from the sound that the original models make when firing. Wikipedia
Barrels: 1, 4 or 8
Calibre: 40 mm (1.6 in)
Effective firing range: 3,960 m (13,300 ft) A/A ceiling
Feed system: 14-round steel-link belt
Filling weight: 71 g (2.5 oz)
In service: 1917–1940s (Mk II); 1930–1940s (Mk VIII)
Length: 8 ft 6 in (2.59 m)