An experimental gun developed in 1931 as a destroyer weapon and also investigated for arming the aircarft carrier HMS Ark Royal. Two prototypes were manufactured, one for shore trials and the other mounted in the "B" position on HMS Kempenfelt, the "C" class flotilla leader. This gun fired fixed ammunition weighing some 108 lbs. (49 kg), an incredibly heavy round for a manually-worked destroyer weapon, and development was abandoned shortly after the completion of the initial trials.
The original projectile weighed 70 lbs. (31.8 kg), although later ones were 62 lbs. (28.1 kg) in an apparent attempt to reduce the complete round weight. Muzzle velocity was 2,693 fps (821 mps) for the 70 lbs. (31.8 kg) projectile and about 2,790 fps (850 mps) for the 62 lbs. (28.1 kg) projectile.
Construction was different in the two prototypes, with one having a monobloc barrel and the other a two piece barrel. The mountings were modified 4.7" (12 cm) CP XIV with a new cradle. As the maximum elevation of this mounting was only 40 degrees, it can be seen that this weapon was intended primarily for surface warfare.
Actual bore diameter was 5.1" (12.954 cm).
The British launched an intensive post-war investigation into new 5" (12.7 cm) guns as part of the Mid-Caliber Dual Purpose (MCDP) program. These were intended to be used to arm the large destroyers projected in the late 1940s and early 1950s. They were fixed ammunition weapons with automatic feed mechanism and used a water jacket. Development may have been a joint effort with the USN 5"/70 (12.7 cm) Type F, similar to the joint program for the 3"/70 (7.62 cm) guns of the same era. Based upon that assumption, these guns would have fired a 70 lbs. (31.8 kg) projectile at about 3,400 fps (1,036 mps). The complete round would have been about 65 inches (165 cm) long and weight about 115 lbs. (52 kg).
In 1948, Vickers-Armstrong (Barrow) submitted a proposal for a twin 5"/70 (12.7 cm) mounting having a revolving weight of 189 tons (192 mt) and 311 tons (316 mt) in total (including 500 rounds of ammunition) while Vickers-Elswick submitted a proposal for a twin 5"/70 (12.7 cm) mounting having a revolving weight of 113 tons (115 mt) and 265 tons (269 mt) in total (including 500 rounds of ammunition). For comparison, the twin 4.5"/45 (11.4 cm) Mark VI mounting used on the Daring class destroyers weighed 44 tons (45 mt). In addition, there was also a single mounting proposal with a maximum elevation of 90 degrees and a total weight of 77 tons (78 mt). Cyclic rate of fire for all of these designs was about 66 rpm, although higher figures are also quoted.
These weights were considered to be too heavy and so the project was changed in November 1951 to a shorter barrel length of 5"/62 (12.7 cm) with a ROF of 60 rounds per minute. This weapon was to have a muzzle velocity of 3,400 fps (1,036 mps) and use fixed ammunition and water cooling. At these very high muzzle velocities, it was expected that the barrel would heat rapidly to unsafe levels. Therefore, firing was limited to 100 rounds at a time, followed by a five minute pause to allow the barrel to cool. Following this pause, an additional 100 rounds could be fired. Two kinds of ammunition were to be developed, an HE round for anti-aircraft fire and a HEP (HE Piercing) for anti-surface fire. HEP was expected to pierce 1 inch (2.54 cm) NC plate at the minimum striking velocities at angles up to 50 degrees. HEP was to have a burster of at least 5 lbs. (2.3 kg).
A 4,700 - 4,800 ton destroyer/cruiser design of 1951 shows three of these guns, mounted bow, amidships and stern. Outfit was to be 270 rounds per gun, a rather surprisingly small amount, as it is equivalent to only four minutes of firing at the maximum ROF.
Again, these mountings were determined to be too heavy and the design was changed once again to a 5"/56 (12.7 cm) weapon, as detailed below.
Based upon a Vickers design for a land anti-aircraft gun, this navalized version was essentially a shorter and thus lighter version of the Mark N1. This gun was to be used in both single and twin mountings. Data on this weapon is sparse, but the single mounting would have weighed about 55 tons (56 mt) while the twin would have been about 92 tons (93 mt). Both mountings had a maximum elevation of 90 degrees. Cyclic rate of fire for this version was reduced to 40 rpm, apparently as part of the effort to reduce the mounting weight. Even this lower rate of fire was sustainable for only the first minute, subsequent firing was limited to no more than 10 rounds per minute in order to extend barrel life. Shell weight was set at 58 lbs. (26.3 kg) and muzzle velocity at 3,200 fps (975 mps).
The light cruiser intended to carry these weapons was cancelled in 1953 and the cancellation of the guns followed soon afterwards. However, the technology already developed was incorporated into an Army medium caliber anti-aircraft gun and then into a proposed naval 4" (10.2 cm) anti-aircraft gun that was to used the same barrel as the 4"/45 (10.2 cm) Mark 16. Although not purchased by the Royal Navy, an improved version of this weapon did find a home on two Chilean destroyers built by Vickers, Almirante Williams and Almirante Riveros.
- "Nelson to Vanguard: Warship Design and Development 1923-1945" by D.K. Brown
- "British Naval Guns 1880-1945 No 14" article by John Campbell in "Warship Volume VIII"
- "The Postwar Naval Revolution" by Norman Friedman
- "Vanguard to Trident: British Naval Policy since World War Two" by Eric J. Grove
- "Warships of World War II" by H.T. Lenton and J.J. Colledge