3.7 inch Anti-Aircraft Gun

3.7 inch Anti-Aircraft Gun

After studying a number of gun designs, in 1928 the RA Committee suggested a 3.7 inch Anti-Aircraft Gun, firing a 25 lb shell to a ceiling of 28,000 feet would fill the gap between the existing 3 inch gun and the 4.7 inch then under development. A specification was issued in 1933 for a 3.7 inch gun weighing 8 tons, capable of being towed at 25 mph and brought into action in 15 minutes. Designs were put forward in 1934 and the Vickers design accepted. Production was authorised in April 1937 and the first guns delivered in January 1938.

The carriage was complicated and it weighed well over the specified 8 tons so, at first, people preferred the 3 inch for being lighter and handier. Its vastly superior performance soon changed peoples minds and by mid war it was being acclaimed as one of the best guns of its type in existence.

It was contiually improved, the most important advance being the Molins Fuze Setter No. 11. This was a combined fuze setter and loading mechanism. The fuse setter descended on the shell, set the fuze and withdrew. The tray swung over to the breech and the round was rammed. The breech closed, tray retracted and the gun fired. The whole operation from putting the shell in the tray and hitting the switch was entirly automatic. This not only took strain off the crew and speeded up the process it also produced a fixed time between fuze setting and firing which aided more accurate prediction of the target.

In January 1941, the War Office demanded a new design with a ceiling of 50,000 feet and a time of flight to that height of 30 seconds. They also required the ability to fire 3 rounds with a fourth loaded in 20 seconds. The proposals for this new design were based around the Naval 5.25 inch gun and the existing 4.5 inch gun. They were the 5.25 as it stood or lined down to 4.5 or lined down to 3.7 and the 4.5 lined down to 3.7. The 5.25 inch was chosen as the long term solution but the 4.5 lined down to 3.7 inch was adopted as a stop gap until sufficient 5.25 inch guns were available.

To deal with the high velocity required, a new rifling was introduced. Known as RD (Research Department) Rifling it worked with a specially designed shell. Rifling started at zero with the lands rising to full height just over 4 inches from the start of rifling. At the muzzle end the grooves gradually reduced until at 11 inches from the muzzle the gun had become a smoothbore. The shell had a driving band and twin centring bands at the shoulder. These spread the stress of the shell spinning, evenly along its length. As the depth of rifling decreased, the copper bands were squeezed into canelures in the body. Instead of protruding into the airstream and degrading the flight, these bands were now smooth with the shell body, improving airflow and maintaining velocity.

This gun was introduced in 1943 as the Mark 6. It had a 65 calibre 3.7 inch liner in the jacket of a 4.5 inch Mark 2 gun on a 4.5 inch static mounting. Although only a stop gap, it performed so well it remained in service until 1959.


Mark 1 1937Loose barrel liner in full length jacket & Mk1 breech mechanism
Mark 1/1 1948Mk1 modified by radiussing rear corners of breech block mortise
Mark 2 1937Loose barrel in short jacket. Mk1 or Mk2 barrel and breech
C Mark 2 1944Canadian made Mk2
Mark 2A 1945Change to breech ring and block to fit Mountings 2B or 3A
Mark 2/2 1948Conversion of Mk2 by radiussing breech ring mortise
Mark 2/3 1948Conversion of Mk2A as for Mk2/2
Mark 2/4 1951Conversion of Mk2/2 or 2/3 for elecric firing gear on Mk4 mountings
Mark 3 1938Mk1 gun with Mk2 breech. Made in limited numbers
Mark 3/1 1948Conversion of Mk3 by radiussing breech ring mortise
Mark 3/2 1947Conversion of Mk3 for use with Mk3A or C3A mounting
Mark 3/3 1947Conversion of Mk3/1 as for Mk3/2
Mark 3/4 1951Conversion of Mk3/1 or 3/2 as for Mk2/4
Mark 4 1943Experimental 3.7 loose liner in 4 inch Mk5** gun body. 3 made, cancelled
Mark 5 1943Experimental 65 calibre 3.7 inch loose liner with orthodox rifling in the
4.5 inch Mk2 gun body. Abandoned in favour of Mk6 design
Mark 6 194365 calibre 3.7 inch loose liner with RD Rifling in 4.5 inch Mk2 gun body
Mark 7 1943Mk2 gun with simplified breech mechanism. Only a few experimental
guns made


Mark 1Mobile, original design. Magslip data dials & rocking bar open sights.
Gunlayers faced forward. Gun trunnioned at rear with spring balancing
gear. Loading tray removed in 1942 & replaced when Molins MFS No.11
fuze setter fitted
Mark 1AAs Mk1 but without rocking bar open sights
Mark 2Static mounting. Fitted with counterbalancing weight instead of spring
balancing gear. Magslip & rocking bar sights, layers faced rear. Fixed to Holdfast
AA Mounting No.2
Mark 2AMk2 fitted with R37 remote power control
Mark 2BMk2 fitted with MFS No.11 & automatic loading gear
Mark 2CMk2 with R37 RPC 7 MFS No.11
Mark 3Wartime production, mobile. Similar to Mk1 but layers faced rear
Mark 3AMk3 with MFS No.11
Mark C3Canadian made Mk3. Carriage, wheels & brakes differed from British version
Mark 44.5 inch Mk1 mounting modified for 3.7 inch Mk6
Mark 4/1Mk4 with MFS No.12 & automatic loading gear




3.7 inch Gun Mks 1-3

Weight of Gun & Breech Mechanism3,931 lbs
Total Length195.15 inches
Length of Bore185 inches (50 calibres)
Rifling28 grooves, uniform right hand 1/30
Breech MechanismHorizontal sliding block, semi-automatic,
percussion fired
Elevation-5° to +80°
Recoil SystemHydropneumatic, constant 32 inches on
mobile mounting.
Hydro-spring, constant 18 inches on static mounting
Weight in Action20,541 lbs mobile; 23,100 lbs static
Rate of FireHand loading 10 rounds per minute
Auto-loading 25 rounds per minute

3.7 inch Gun Mk 6

Weight of Gun & Breech Mechanism6,552 lbs
Total Length252 inches
Length of Bore240.5 inches (65 calibres)
Rifling28 grooves, uniform right hand 1/27, RD system
Breech MechanismHorizontal sliding block, semi-automatic,
percussion fired
Elevation0° to +80°
Recoil SystemHydropneumatic, constant 18 inches
Weight in Action38,360 lbs
Rate of FireHand loading 8 rounds per minute
Auto-loading 19 rounds per minute


3.7 inch Gun Mks 1-3 firing standard 28 lb HE Shell

Muzzle Velocity2,600 feet/second
Maximum Horizontal Range20,600 yards
Maximum Ceiling41,000 feet
Effective CeilingVaried with fuze, Predictor & method of fire control
32,000 feet with Predictor No.10 & Fuze Time No. 208


Shell, HE, Mark 1CUsed Fuze, Time 119 or 223 earlyn on, then Mechanical
Time Fuze 207, 208 or 214
Shell, Shrapnel, Mark 2CFor use against low flying aircraft. Used Fuze Time No. 199
Shot , AP, Mark 5TSolid steel shot with a tracer. Penetration of
117mm/1,000 yards/30°
Propelling ChargeFull Charge – standard charge
Reduced Charge – training and ground role use
Burst-short Charge – practice against live targets. Follows
the same flight path as Full charge but bursts well short
of the target for safety

3.7 inch Gun Mk 6 firing standard 28 lb HE Shell

Muzzle Velocity3,425 feet/second
25,600 yards
59,300 feet
Effective Ceiling45,000 feet with Predictor No.10 & Fuze Time No. 208


Shell, HE, Mark 4CSimilar to Mk1C above but with forward centering bands
& wider driving band. Used with Time Fuze 218
Propelling ChargeOnly the Full Charge was provided
The Royal Artillery 1939-45