QF 13 pounder 9 cwt

QF 13 pounder 9 cwt anti-aircraft gun
on Mark IV mounting on Peerless lorries at Cambrin, 13 March 1918
Type	Anti-aircraft gun
Place of origin	United Kingdom
Service history
In service	1915–1920s
Used by	British Empire
Wars	World War I
Specifications
Weight	7.5 tons
Barrel length	Bore: 7 feet 9 inches (2.36 m); Total: 8 feet 1 inch (2.46 m)
Shell	12.5 pounds (5.67 kg) Shrapnel; later HE
Calibre	3-inch (76.2 mm)
Recoil	Hydro-spring, constant 24 inches (610 mm) (Mk III mount); 35 inches (889 mm) (Mk IV mount)
Carriage	high-angle mounting on lorry
Elevation	0°–80°
Traverse	360°
Rate of fire	8 rds/min
Muzzle velocity	2,150 ft/s (660 m/s)
Maximum firing range	19,000 ft (5,800 m)

The 13 pounder 9 cwt anti-aircraft gun became the standard mobile British anti-aircraft gun of the World War I era, especially in theatres outside Britain.

Earlier anti-aircraft guns based on 13 pounder and 18 pounder guns proved unsatisfactory, primarily due to their low muzzle velocities. On 18 February 1915 Sir John French, commander of the British Expeditionary Force in France, asked for an anti-aircraft gun with a muzzle velocity of 2,000 feet per second (610 m/s). On 19 August 1915 the Army Council proposed adapting existing 18-pounder guns (3.3-inch bore) to use 13-pounder (3-inch) shells, thus meeting the requirement for higher velocity.

This weapon combined an 18 pounder breech and barrel with a liner (sleeve) inserted to reduce the bore from 3.3-inch (84 mm) to 3-inch (76 mm) so that it could fire the slightly smaller 13 pounder shell but still use the larger cartridge and propellant charge of the 18 pounder resulting in a much higher velocity. A slight neck was introduced in the 18 pounder cartridge to hold the slightly narrower 13 pounder shell in place.

The initial Mk III mounting was based on the 13 pounder Mk II anti-aircraft mounting, but proved to be not strong enough for the extra power of the 18 pounder cartridge.

The Mk IV mounting which followed raised the height by 9 inches (230 mm) and increased recoil from 24 to 35 inches (890 mm) and hence relieved the strain on the mounting.

Several guns are known to have been mounted on 2-wheeled high-angle field carriages and deployed on the Italian front. Hogg & Thurston state that they could theoretically be used as anti-aircraft guns, field guns or howitzers, but they were not officially introduced and may have been of an experimental nature. Routledge states that the carriage was improvised because some of 4th AA Group's guns had arrived in Italy without mountings.

As World War I progressed, it was replaced in the home air defence of England (against German heavy bombers) by the more powerful QF 3 inch 20 cwt gun, but continued in all other theatres. It was usually deployed mounted on medium lorries such as the Thornycroft Type J with a speed of 18 miles per hour, in sections of 2 guns.