“Meanwhile, great effort was being expanded on the possibility of increasing the Whirlwind’s armament by adding three 0.303in Browning machine-guns to the four nose mounted 20mm cannon. It was also proposed that the ammunition drums be replaced by a belt-fed system. In an exercise to cover the possible failure or lack of the Hispano cannon and ammunition, a quickly replaceable 12-machine-gun “weapon egg”, complete with ammunition boxes, was designed and produced by Martin-Baker to replace the four-cannon pack. There were experimental installations of a single nose mounted cannon and a four-in-a-row configuration. These were flown in L6844 at the RAE as trial installations of a belt-feed mechanism.

Much has been said about the calibre of the single-cannon installation, “armament experts” arguing over the well-known photograph purporting to show a 40mm cannon. Penrose thought that 37mm cannon might have been installed but not necessarily flown. In fact this single weapon was a Hispano Mk II 20mm cannon with a Westland-designed pneumatically-powered ammunition feed system”.

Derek James Westland Whirlwind Database The Aeroplane May 2006