QF 12-pounder 12 cwt naval gun

As first mounted on 27-knot destroyers from 1894, here seen on HMS Daring

Mk V gun on a British trawler, World War II

Mk I and II guns, of "built up" construction of multiple steel layers, served on many Royal Navy destroyers up to and after World War I originally as primary and later as secondary armament against submarines and torpedo boats. They were also fitted as deck guns on D and E-class submarines.

It was estimated that out of the 4,737 Mk I and Mk II guns produced there were still 3,494 on hand for the RN in 1939.[4] Many Mk V guns, which had a "monobloc" barrel made of a single casting, served on smaller escort ships such as destroyers and on armed merchant ships, on dual-purpose high-low angle mountings which also allowed it to be used as an anti-aircraft gun.

Naval brigade with a "long twelve" in Natal

Elswick Battery gun in South Africa

The gun was primarily a high-velocity naval gun, with its heavy recoil suiting it to static mountings, hence it was generally considered unsuitable for use as a mobile field gun.[5] An exception was made when the British army were outgunned by the Boer artillery in South Africa and the Royal Navy was called on for help. Among other guns, 16 QF 12-pounder 12 cwt were landed from warships and were mounted on improvised field carriages designed by Captain Percy Scott RN, with solid wooden trails and utilizing small-diameter Cape wagon wheels. Their 10,000-yard (9,100 m) range provided valuable long-range fire support for the army throughout the war. They were known as "long twelves" to distinguish them from the BL 12-pounder 6 cwt and QF 12-pounder 8 cwt which had much shorter barrels and ranges.[6]

Lieutenant Burne reported that the original electric firing system, while working well under ideal conditions, required support of an armourer and the maintenance and transport of charged batteries in the field, which was generally not possible. He reported switching to percussion tubes for firing and recommended percussion for future field operations.[7]

Another six guns were diverted from a Japanese battleship being built at Newcastle in January 1900, bought by Lady Meux, and were equipped with proper field carriages by the Elswick Ordnance Company in Newcastle and sent to South Africa. Perhaps uniquely, the guns were refused by the War Office and donated directly to Lord Roberts, the British commander in South Africa and became his personal property. They were known as the "Elswick Battery" and were manned by men from Elswick, recruited by 1st Northumberland Royal Garrison Artillery (Volunteers). The Elswick guns served throughout the war.[8]

Typical coast defence mounting, at Newhaven Fort, UK

Many guns were mounted on "pedestals" secured to the ground to defend harbours around the UK, and at many ports around the Empire, against possible attack by small fast vessels such as torpedo boats, until the 1950s. There were 103 of these guns (of a total 383 of all types) employed in coast defence around the UK as at April 1918.[9] Many of these were still in service in World War II although they had by then been superseded by more modern types such as twin QF 6 pounder 10 cwt mounts.

Guns were traversed (moved from side to side) manually by the gunlayer as he stood on the left side with his arm hooked over a shoulder piece as he aimed, while he operated the elevating handwheel with his left hand and grasped the pistol grip with trigger in his right hand.[5]

In World War I a number of coast defence guns were modified and mounted on special wheeled traveling carriages to create a marginally effective mobile anti-aircraft gun.

UK shells weighed 12.5 lb (5.67 kg) filled and fuzed.
The cordite propellant charge was normally ignited by an electrically activated primer (in the base of the cartridge case), with power provided by a battery. The electric primer in the cartridge could be replaced by an adaptor which allowed the use of electric or percussion tube to be inserted to provide ignition.

2 lb Cordite cartridges Mk II & Mk III, 1914	Mk II common pointed shell	Mk III & Mk II common Lyddite shell	Mk IV common Lyddite shell with internal night tracer, 1914	Mk IX shrapnel shell, 1914

• Text Book of Gunnery, 1902. LONDON : PRINTED FOR HIS MAJESTY'S STATIONERY OFFICE, BY HARRISON AND SONS, ST. MARTIN'S LANE
• Brown, D. K. (2003). Warrior to Dreadnought: Warship Development 1860–1905. Book Sales. ISBN 978-1-84067-529-0.
• Brown, D. K. (2003). The Grand Fleet: Warship Design and Development 1906–1922. Caxton Editions. ISBN 978-1-84067-531-3.
• Lieutenant C. R. N. Burne R.N., With the Naval Brigade in Natal (1899–1900). London: Edward Arnold, 1902
• Farndale, General Sir Martin (1988). History of the Royal Regiment of Artillery. The Forgotten Fronts and the Home Base, 1914–18. Royal Artillery Institution, London. ISBN 978-1-870114-05-9.
• Gardiner, Robert; Lambert, Andrew, eds. (2001). Steam, Steel and Shellfire: The Steam Warship, 1815–1905. Conway's History of the Ship. Book Sales. ISBN 978-0-7858-1413-9.
• Hodges, Peter (1981). The Big Gun: Battleship Main Armament, 1860–1945. United States Naval Institute Press. ISBN 978-0-87021-917-7.
• Hogg, I.V. and Thurston, L.F. (1972). British Artillery Weapons & Ammunition 1914–1918. Ian Allan, London. ISBN 978-0-7110-0381-1.
• Parkes, Oscar (1990) [1957]. British Battleships. United States Naval Institute Press. ISBN 978-1-55750-075-5.
• Admiral Percy Scott, "Fifty Years in the Royal Navy" published 1919
• Campbell, John (1985). Naval Weapons of World War Two. Naval Institute Press. ISBN 978-0-87021-459-2.