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. 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.
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. 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.
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.