4.5 inch Medium Gun

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 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 4.5’s and one battery of 5.5’s.

Mk 1Conversion 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 2New 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.1Detachment Commander
No.2Breech Operator – fires gun
No.3Layer
No.4Load & Ram
No.5Load & Ram
No.6Prepare cartridges
No.7Prepare shells
No.8Prepare shells
No.9Prepare shells
No.10Coverer, supervise ammunition

Data

Gun Mk2 on Carriage Mk1 or 2

Weight of gun & breech mechanism4,263 lbs
Total length192.75 inches
Length of bore185 inches (41 calibres)
Rifling32 grooves, uniform right hand 1/25
Breech mechanismAsbury interrupted screw, percussion fired
Elevation– 5° to +45°
Traverse30° left & right
Recoil systemHydropneumatic, variable, 30 inches to 54 inches
Weight in action12,880 lbs

Performance

Firing 55 lb HE shell

Muzzle Velocity2,250 feet/second
Maximum Range20,500 yards

Ammunition

Separate loading, bag charge

Shell, HE, S/L MK 1DThis was the only projectile issued for the gun. Used
Fuze, Percussion No. 117 or 119.
Propelling ChargeThis 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.