where were they used?
It was used in the Second Opium War.
Armstrong gun
The British used Armstrong guns extensively to great effect in the Second Opium War. 
what is cragside? A Victorian country house. where is it located? Near the town of Rothbury in Northumberland. how is the house and armstrong connected? It was the home of William Armstrong, 1st Baron Armstrong. what is the architectural style of the house? Tudor revival style. what was armstrong's profession? An engineer and an industrialist. was he supportive of weapons? He was the founder of an armament firm and supported weapons. has he designed any weapon? Yes, the Armstrong gun.
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Armstrong gun [SEP] Armstrong guns in action
The British used Armstrong guns extensively to great effect in the Second Opium War. As reported by the translator Robert Swinhoe, after the British attack on the Chinese fort at Pehtang: Numbers of dead Chinese lay about the guns, some most fearfully lacerated. The wall afforded very little protection to the Tartar gunners, and it was astonishing how they managed to stand so long against the destructive fire that our Armstrongs poured on them; but I observed, in more instances than one, that the unfortunate creatures had been tied to the guns by the legs. The Armstrong gun—mainly the 12 pounder—was used extensively in the 1863 conflict in New Zealand between British troops and Maori in the Waikato.