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).

See 5.1

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).

Artist conception of 1953 Cruiser Design with two bow 5"/62 (12.7 cm) twin mountings
The hull openings are said to have been the result of erroneous information given to the artist
Painting by Peter Hogan

  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.