In 1933 it was proposed as a modification of the 60 pdr Gun with a more modern design shell and greater range. Due to financial restrictions, it ended up as just relining the barrel. The conversion was tested in 1937 and approval given. It was then found that only 76 60 pdrs were available for conversion. In 1938 the Director RA authorised, on his own initiative, development of a new gun, the 4.5 inch Medium Gun, to fit the carriage being designed for the 5.5 inch Gun.
This gun, the Mark 2, was approved in August 1939 but production did not start until late 1940. The guns were not issued until 1941. They were identical to the 5.5 inch Gun except for calibre and barrel length (the 4.5 was some 2 feet longer). They were issued to Medium Regiments, generally as one battery of the 4.5 inch Medium Gun and one battery of 5.5’s.
| Mk 1 | Conversion of 60 pdr Mk 2 or Mk 2* by relining |
| Mk 1* | Conversion of 60 pdr Mk 2 or Mk 2* by insertion of an auto-frettaged loose barrel into the jacket |
| Mk 2 | New design to fit the 5.5 inch carriage |
Carriage
| Marks 1 & 2 (only differences are a change from rivetted to welded construction) | Split trail with hydropneumatic recoil system. Hydropneumatic balancing cylinders in 2 vertical mounts to balance the barrel mass. Because of leakage problems these were changed to springs. Trough cradle with trunnions towards the rear to allow recoil at full elevation. Quick loading gear. |
Crew
| No.1 | Detachment Commander |
| No.2 | Breech Operator – fires gun |
| No.3 | Layer |
| No.4 | Load & Ram |
| No.5 | Load & Ram |
| No.6 | Prepare cartridges |
| No.7 | Prepare shells |
| No.8 | Prepare shells |
| No.9 | Prepare shells |
| No.10 | Coverer, supervise ammunition |
Data
Gun Mk2 on Carriage Mk1 or 2
| Weight of gun & breech mechanism | 4,263 lbs |
| Total length | 192.75 inches |
| Length of bore | 185 inches (41 calibres) |
| Rifling | 32 grooves, uniform right hand 1/25 |
| Breech mechanism | Asbury interrupted screw, percussion fired |
| Elevation | – 5° to +45° |
| Traverse | 30° left & right |
| Recoil system | Hydropneumatic, variable, 30 inches to 54 inches |
| Weight in action | 12,880 lbs |
Performance
Firing 55 lb HE shell
| Muzzle Velocity | 2,250 feet/second |
| Maximum Range | 20,500 yards |
Ammunition
Separate loading, bag charge
| Shell, HE, S/L MK 1D | This was the only projectile issued for the gun. Used Fuze, Percussion No. 117 or 119. |
| Propelling Charge | This was produced in 2 forms, differing in the shape of the bags. Charges 1 & 2 were used separately & Charge 3 was an increment charge for Charge 2. |