4 . 7 inch QF Mark IX & XII The 4 . 7 inch QF Mark IX and Mark XII were 120 - mm 45 - calibre naval gun s which armed the majority of Royal Navy and Commonwealth destroyer s in World War II , and were exported to many countries after World War II as the destroyers they were mounted on were sold off . Description and history . These guns succeeded the similar World War I - era BL 4 . 7 inch gun , changing the cartridges from BL silk bags to separate QF in brass cases and a new horizontal sliding block breech mechanism . Mark IX was deployed in single mountings CP Mk XIV on the A class destroyers of 1930 and on most subsequent destroyer classes up to and including the R class of 1942 . The almost - identical Mk XII gun was deployed in twin mountings CP Mk XIX on the Tribal class destroyers of 1936 and J , K and N classes of 1938 . This mounting limited the maximum elevation to 40 degrees , but all twin CP Mk XIX were dual - purpose mountings and were equipped with " Fuze Setting Pedestals " or " Mk V Fuze Setting Trays " , to allow the mountings to be fired against aircraft while being controlled by the Fuze Keeping Clock fire control computer . Typical maximum rate of fire was twelve rounds per gun , per minute . During gunnery trials in 1930 , HMS " Basilisk ' ' was able to fire " . . . five rounds in 17 seconds . " The Mk XII gun fired a 50 lb shell and used a separate cartridge , with both shell and cartridge being loaded via a loading tray , with power ramming , elevation , and traverse . The maximum range at 40 degrees elevation was 16 , 970 yards ( 15520m ) fired at the new gun muzzle velocity of 2650fps ( 808 m / s ) . The 40 degree elevation was justified on the grounds that destroyers would be screening the battle - fleet during aerial attack , and 40 degrees elevation was adequate to engage aircraft that were concentrating their attack on other ships . Admiral Sir Philip Vian describes the use of Tribal Class destroyer mounted Mk XII guns against aircraft during the campaign in Norwegian waters , April through June 1940 : " It became clear at once that in an attack from the air in narrow waters flanked by mountains , the cards were held by the aircraft . There was too little sea - room for full freedom of manoeuvre , and the aircraft ' s approach was screened by the rock walls . As often as not , when they did come into view it was at such an angle that our 4 . 7 - inch guns , whose maximum elevation was only forty degrees , could not reach them . . . Aandalsnes is approached through the Romsdal Fiord , and lies forty miles from the entrance , off which we arrived on the 24th April . The daylight passage of the convoy and escort through this waterway , speed five knots , on a steady course and with mountains rising steeply either side , presented an alluring invitation to enemy aircraft . Junkers attacks persisted to the end , but the fire of the destroyers , although limited to an elevation of forty degrees , was enough to keep the enemy just too high for their standard of marksmanship . Not a ship received a direct hit , though some were damaged by the splinters from near misses . " The S class , introduced the CP ( central pivot ) single Mark XXII mounting for the QF Mark XII 4 . 7 in gun . This new mounting had a shield with a sharply raked front , to allow increased elevation ( to 55 degrees ) , contrasting noticeably with the vertical front of the previous CP Mark XVIII , and easily differentiated the S class onwards from their immediate predecessors . The " Savage " was the exception in this respect , being fitted with 4 . 5 inch calibre ; a twin mounting forward and two singles aft . The 4 . 7 inch calibre was finally superseded by the 4 . 5 inch calibre on the Z class destroyer s in 1943 . The new 4 . 5 inch guns all had 55 degree elevation mounts and actually fired a shell slightly heavier than that of 4 . 7 - inch Mk IX and XII guns , although slightly lighter than that fired by the 4 . 7 inch Mk XI gun .