3 inch 20 cwt Gun

3 inch 20 cwt gun

Under the sponsorship of the Royal Navy, the 3 inch 20 cwt Gun began development prior to World War 1. Both the Royal Navy and the Army accepted it for service in 1914 as the standard anti-aircraft gun. The original mounting was a static pedestal for anchoring in concrete. Mobile mountings came into use later. Sighting was direct with open sights. In the late 1930’s, some guns had data-recieving dials fitted. They could then be under predictor control. It became obsolete in 1946.


Mark-1Wire wound, vertical sliding breech, rifled 1/30
Mark-1*As Mark-1 but rifled 1/40. Later re-lined & rifled 1/30
Mark-1**Mark-3 converted back to Mark-1 breech mechanism in 1923
Mark-1***Mark-3* converted back to Mark-1 breech mechanism in 1923
Mark-1AMarks-1 to 1*** repaired by removing “A” tube & inserting auto-frettaged loose liner in 1933
Mark-1BNew manufacture with auto-frettaged loose liner, 1933
Mark-1DNaval. Auto-frettaged monobloc barrel, 1941
Mark-1ENaval. Monobloc barrel of high-yield steel, 1941
Mark-2Naval. As Mark-1 but differences in breech ring to suit naval mounting
Mark-2*As Mark-2 but rifled 1/40, 1918
Mark-2CMark-2 repaired as for Mark-1A
Mark-3As for Mark-1 but with screw breech mechanism, 1917
Mark-3*As Mark-3 but rifled 1/40, 1918
Mark-3AMark-3 or 3* repaired as for Mark-1A, 1933
Mark-4Monobloc gun with Wellin screw breech, 1916. Obsolete 1926
Mark-4*As Mark-4 but rifled 1/40

Carriage & Mountings

Mark-1Pedestal, variable recoil, 2 speed elevating gear
Mark-2As Mark-1 but 1 speed elevating gear, constant length recoil
Mark-3Simplified Mark-2 for war time production 1916
Mark-4Added run out control for semi-automatic breech
Mark-4AWithout run out control for screw breech guns
Lorry Peerless AACommercial truck carrying Mark-4 mount. Solid tyres, fring support jacks
Platform-2 wheeled Mark-1Square platform, single axle, short outriggers, 1916. Pneumatic tyres fitted 1938
Platform-4 wheeled Marks 1 & 2Pneumatic tyres, cone jacks for levelling & support, 1928
Platform-4 wheeled Mark-3As Mark-2 but with mechanical 4-wheel brakes
Platform-4 wheeled Mark-3AAs Mark-2 but with Warner Electric brakes
Platform-4 wheeled Mark-3BAs Mark-3A but different pattern of Warner brakes
Platform-4 wheeled Marks 4, 4A & 4BSimilar to Marks 3, 3A & 3B but with different jacking system


Gun Mark-1 on Mounting Mark-1

Weight of gun & breech mechanism2,250 lbs
Total length140 inches
Length of bore135 inches (45 calibres)
Rifling20 grooves, uniform, right hand, 1/30
Breech mechanismVertical sliding block, semi-automatic, percussion fired
Elevation-10° to +90°
Recoil systemHydro-spring, variable, 11 to 20 inches
Weight in action6,000 lbs
Rate of fire20 – 25 rounds per minute


Firing standard 16.5 lb HE Shell

Muzzle velocity2,000 feet/second
Maximum horizontal range12,400 yards
Maximum ceiling25,200 feet
Effective ceiling15,700 feet
An earlier pattern of 12.5 lb shell gave the following performance
Muzzle velocity2,500 feet/second
Maximum horizontal range10,900 yards
Maximum ceiling37,200 feet
Effective ceiling23,500 feet


Fixed case charge

Over its service, the gun had many different varieties of ammunition issued. A list for WW1 shows 215 combinations of charge & shell. An inventory from 1943 still lists 66 different complete rounds in service. The list below, therefore, is a representative one only.

Shell, HE, Mark-2BFlat based shell with Time Fuze No. 199
Shell, Shrapnel, Mark-2BTBase tracer, Time Fuze No. 199. For low flying aircraft
Shot, AP, Mark-2TIntegral tracer. Penetration 84mm at 1,00 yards & 30° angle
Propelling chargeBrass case, 2 lb 2 oz of Cordite with Precussion Primer No. 1
The Royal Artillery 1939-45