Royal Navy service
As the QF 6 inch Mk I, Mk II and Mk III, the gun was used as secondary armament of pre-dreadnoughts of the 1890s and cruisers to 1905. On the armoured cruisers of the Diadem, Powerful and Edgar classes they made up most of the armament, though the latter class carried two 9.2-inch (230 mm) guns as well. The pre-dreadnought battleships of the Royal Sovereign (including the turreted HMS Hood), Centurion, Majestic and Canopus classes carried up to 12 guns.
Second Boer War land service
During the Second Boer War one gun was brought ashore from HMS Terrible in Natal in February 1900 at the request of General Buller, presumably in response to the failure at Colenso. It was mounted on an improvised field carriage by Captain Percy Scott and transported by rail to Chieveley, just south of Colenso. There it was manned by Royal Navy gunners to provide useful fire support for the British Army during the relief of Ladysmith. It is reported on 17 February to have fired from "Gun Hill" (a small kopje 2 two miles (3 km) north of Chieveley) and knocked out a Boer gun at 16,500 yards (15,100 m), followed by a Boer searchlight, as Buller approached Ladysmith from the South East and pushed the Boers back towards the Tugela river. On 26 February Lieutenant Burne reports firing from the same position on a Boer gun at 15,000 yards (14,000 m) at 28° elevation and falling 200 yards (180 m) short. The 7 ton weight (compared to the 2½ tons of the Boer 155 mm "Long Tom") meant that it was effectively immobile on the battlefield and could not be moved forward to shorten the range.
Two guns were also mounted on armoured trains, crewed by Royal Garrison Artillery men.
Coast defence gun
From 1894 a number of guns were adapted for coast defence use, with the original 3-motion breeches replaced by modern single-motion breeches to increase the rate of fire, which designated them as "B" guns.
Nineteen guns were still active in the defence of the UK as at April 1918 : Jersey (2), Guernsey (2), Alderney (2), Shoeburyness (2), Blyth (2), Clyde Garrison (1), Mersey (2), Berehaven Garrison (Bantry Bay, Ireland) (6).
World War I anti-aircraft gun
At least one gun is known to have been mounted by the Royal Navy on an improvised anti-aircraft mounting on a railway truck, defending docks during the First World War.
Conversion to 8 inch (203 mm) howitzer
In World War I Britain urgently needed heavy artillery on the Western Front, and various obsolete 6-inch naval guns were converted to 8-inch howitzers. Sixty-three QF 6-inch Mk II guns were shortened, bored out to 8 inches (203 mm) and converted to BL type to produce the BL 8-inch howitzer Mk V. Four entered service in December 1915 and 59 followed in 1916.
Japanese naval service
The Type 41 naval gun was designed by Armstrong Whitworth, Elswick, Newcastle upon Tyne, England as a slightly modified version of the Elswick Ordnance Company QF pattern 6-inch (150 mm) guns used on contemporary Royal Navy battleships. It was the standard secondary armament on early Japanese battleships and the main battery on several classes of armoured cruisers.
The gun was officially designated as “Type 41” from the 41st year of the reign of Emperor Meiji on 25 December 1908. It was further re-designated in centimeters on 5 October 1917 as part of the standardization process for the Imperial Japanese Navy converting to the metric system.
The Type 41 6-inch (150 mm) gun fired a 100-pound (45.4 kg) shell with either an armour piercing, high explosive or general purpose warhead. An anti-submarine shell of 113-pound (51.3 kg) was developed and in service from 1943.
These guns were adopted in very limited quantity by the United States Army Coast Artillery Corps as part of the Endicott period fortifications constructed 1895-1905. They were referred to as Armstrong guns and mounted on pedestals in US service, and appear to have been withdrawn from service by 1925. A total of 13 guns were mounted, 9 in the US and 4 in Hawaii, in one-, two-, or three-gun batteries. They were at Fort Williams (Maine) (1), Fort Adams, Rhode Island (3), Fort Wadsworth, Staten Island, NY (2), Fort Screven, Tybee Island, Georgia (1), Fort Dade, Tampa Bay, FL (2), and Ford Island, Pearl Harbor, Hawaii (4). The two from Fort Dade survive at Fort DeSoto near St. Petersburg, Florida. Their battery at Fort Dade has succumbed to tide action over the years.