The 5.5 became the principal Commonwealth medium artillery piece during World War 2. Artillery was then designated 'field' 'medium' or 'heavy'. The term 'field' included guns up to and including the 4.5-inch howitzer, while medium covered those from the 4.5-inch gun up to and including the 6-inch howitzer. Guns of 6-inch calibre or above were 'heavy'. During World War 1 the principal medium equipments were the BL 60-pr (5-inch) gun and the BL 6-inch 26-cwt howitzer. The former remained in service until 1941, the latter until 1945.
The Royal Artillery had seen the need to update these equipments during the 1930s but were handicapped by lack of finance. To replace the 60-pr a new BL 4.5-inch gun was produced in 1937 but mounted upon the 60-pr carriage fitted with pneumatic wheels. A total of 76 of these were produced, 32 of which were lost in France in 1940. The rest saw service in the Middle East up to 1941 or were used for training purposes. A Mk 2 gun on a new split-trail carriage, a completely new 4.5 equipment, saw service from 1941 until 1945.
The 4.5 achieved a satisfactory range of 20,500 yards (18,758 metres) but its shell lacked power so a BL 5.5-inch gun was produced and mounted upon the same carriage as the 4.5 Mk 2 gun. See Fig 1. It entered service in 1941 and survived until the late 1970s when it was superseded by the 155-mm FH 70 (field howitzer 1970), a NATO equipment, a joint UK-German-Italian project.
So successful had the 6-in howitzer HP recoil system proved that it was incorporated in the 4.5/5.5 carriage virtually unchanged.
No BL 4.5-in guns ever came to New Zealand; the 60-pr remained in use until after World War 2. New Zealand obtained BL 5.5 equipments after World War 2 and retained them until the 1980s but did not replace them. It is believed M198 155-mm howitzers were about to be ordered from the United States but a change of Government in 1984 saw the order cancelled. Instead our medium Regiment, 4 Medium Regiment RNZA, was relegated to the field role and eventually equipped with QF 105-mm 'light' guns.
In the Royal Artillery the BL 5.5 gun was superseded by the 155-mm FH 70 (field howitzer 1970), a NATO equipment produced as a combined effort by Britain, Germany, and Italy. It entered service in 1979.