QF 3 Inch 20 Cwt
The QF 3 inch 20 cwt anti-aircraft gun became the standard anti-aircraft gun used in the home defence of the United Kingdom against German airships and bombers and on the Western Front in World War I. It was also common on British warships in World War I and submarines in World War II. 20 cwt referred to the weight of the barrel and breech, to differentiate it from other "3 inch" guns (1 cwt = 1 hundredweight = 112 lb, hence the barrel and breech together weighed 2250 lb). While other AA guns also had a bore of 3 inches, the term "3 inch" was only ever used to identify this gun in the World War I era, and hence this is what writers are usually referring to by "3 inch AA gun".