On the blog of respected French author Jacques Moulin, he has posted a set of photographs of an extempore armament modification to a Hawker Hurricane, apparently in Egypt.
This mod is new to me, and we can assume it was not successful. While it is obviously undesirably draggy the detail work, particularly the teardrop fairing for the ammunition drums and the aft pillar mount is well developed.
The gun is the Hispano-Suiza HS.404, usually known as the Hispano 20mm, and widely used from about 1940 on British (and other nations') aircraft. The Hurricane went on to get a set of four of these cannon replacing the eight machine guns in the 'c' version of the Mk.II, probably one reason this version didn't prosper. What's not clear is if the eight guns on this machine were retained - the open wing ports imply that they were.
I can't help but wonder if the cannon mounts and set-up are actually a set of the made but rarely used Westland Lysander cannons. We had an interesting discussion of these here on the Key forum and the image shows how they were attached to the Lysander - and the mount similarity, although obviously rotated into a different position. Note the extra bracing on the cannon.
The early Hispano cannon had major problems with the drum feed when it was either moved from the dorsal position as seen here, or under high-g manoeuvring, as found on the first cannon-armed Spitfires in 1940.
(Hurricane photos via Jacques Moulin, credited 'RAF'. Thanks to Jacques for sharing these photos, as originally posted here.)