RA 1939-45 3.7 inch AA gun

Mk 1 1937 - Loose barrel liner in full length jacket & Mk 1 breech mechanism
Mk 1/1 1948 - As Mk 1 with modifications to breech block
Mk 2 1937 - Loose barrel in short jacket. Mk 1 or 2 breech mechanism.
C Mk 2 1944 - Canadian built Mk 2
Mk 2A 1945 - Mk 2 adapted to suit mountings 2B or 3A
Mk 2/2 1948 - Mk 2 with modified breech ring mortise
Mk 2/3 1948 - Conversion of Mk 2A as for Mk 2/2
Mk 2/4 1951 - Conversion of Mk 2/2 or 2/3 for electric firing gear to suit Mk 4 mounting
Mk 3 1938 - Mk 1 gun with Mk 2 breech. Limited numbers
Mk 3/1 1948 - Mk 3 with modified breech ring mortise
Mk 3/2 1947 - Mk 3 conversion for use with Mk 3A or C 3A mounting
Mk 3/3 1947 - Conversion of Mk 3/1 as for Mk 3/2
Mk 3/4 1951 - Conversion of Mk 3/1 or 3/2 as for Mk 2/4
Mk 4 1943 - Experimental. 3.7 loose liner in to 4 inch Mk 5** body. 3 made, project cancelled
Mk 5 1943 - Experimental. 3.7 loose liner into 4.5 inch Mk 2 body. None issued for service
Mk 6 1943 - 3.7 loose liner with RD Rifling into 4.5 inch Mk 2 body
Mk 7 1943 - Mk 2 gun with simplified breech. Experimental guns only

Wt gun & breech mech - 3,931 lb
Wt in action - 20,541 lb mobile: 23,100 lb static
Total length - 195.15 inches
Length of bore - 185 inches
Rifling - 28 grooves uniform RH 1/30
Breech mech - Horizontal sliding block, semi-auto, percussion fired
Elevation - minus 5º to plus 80º
Traverse - 360º
Recoil system - Hydropneumatic, mobile: Hydrospring, static
Recoil length - constant 32 inches, mobile: constant 18 inches, static
Rate of Fire - Hand loading 10 rpm: Auto loading 25 rpm

Wt gun & breech mech - 6,552 lb
Wt in action - 38,360 lb
Total length - 252 inches
Length of bore - 240.5 inches
Rifling - 28 grooves uniform RH 1/27 RD system
Breech mech - Horizontal sliding block, semi-auto, percussion fired
Elevation - 0º to plus 80º
Traverse - 360º
Recoil system - Hydropneumatic
Recoil length - constant 18 inches
Rate of Fire - Hand loading 8 rpm: Auto loading 19 rpm

Production was authorised in April 1937 and the first guns delivered in January 1938. The gun was constantly improved with the most important advance being the adoption of the Molins Fuze Setter which automatically set the fuze length and then loaded and fired the shell. Remote power control was introduced in 1944 and this along with advanced radar, predictors and proximity fuzes gave the gun a 82% sucess rate against the V1 missile. The gun was declared obsolete in 1959 when Heavy AA was replaced by missiles.

Mks 1 -3 with 28 lb HE shell:
Muzzle velocity - 2,600 ft/sec
Max horizontal range - 20,600 yds
Maximum ceiling - 41,000 feet
Effective ceiling - Varied with fuze, predictor and fire control method. With Predictor No.10 & Fuze Time No.208 - 32,000 feet

Mk 6 with 28 lb HE shell:
Muzzle velocity - 3,425 ft/sec
Max horizontal range - 25,600 yds
Maximum ceiling - 59,300 feet
Effective ceiling - With Predictor No.10 & Fuze Time No.208 - 45,000 feet

Mk 1 - Original mobile design. Mag-slip data dials and rocking bar open sights. Gun layers faced forwards. Fitted with loading tray, removed in 1942 and refitted for Molins Fuze Setter
Mk 1A - As Mk 1 but without open sights
Mk 2 - Static mounting. Fitted with counter balance weight instead of springs. Gun layers face rear. Fixed to Holdfast AA Mounting Mk 2. Carried on Limber Transporting 3.7 inch Mk 2 and 4.5 inch Equipments Mk 1
Mk 2A - Mk 2 with R37 Remote Power Control
Mk 2B - Mk 2 fitted with Molins Fuze Setter No. 11, automatic loading gear
Mk 2C - Mk 2 with R37 RPC and MFS No. 11
Mk 3 - Wartime production, mobile. As Mk 1A but with layers facing to rear
Mk C3 - Canadian made Mk 3. Carriage, wheels & brakes differed from British version
Mk 4 - 4.5 inch mounting modified for use with 3.7 Mk 6 gun
Mk 4/1 - Mk 4 with MFS No. 12 & automatic loading gear

Shell HE Mk 1C - 28 lb with Fuze Time 199 or 223 and later Mechanical Time Fuze 207, 208 or 214
Shell Shrapnel Mk 2C - For use against low flying aircraft close to gun position
Shot AP MK 5T - Solid steel shot with tracer

Propelling Charge
In brass, necked case 26.56 inches long.