4 . 5 inch Mark 8 naval gun The 4 . 5 inch Mark 8 is a British naval gun system which currently equips the Royal Navy ' s destroyers and frigates , and some British destroyers and frigates sold to other countries . Background . The 4 . 5 inch gun has been the standard medium - gun calibre of the Royal Navy for use against surface , aircraft and shore targets since 1938 . The current 55 - calibre Mark 8 gun replaced the World War II era 45 - calibre QF 4 . 5 inch Mk I - V naval gun s . Like all British 4 . 5 inch naval guns , it actually has a calibre of 4 . 45 inches ( 113 mm ) . Design . A completely new type of 4 . 5 inch gun with a longer 55 - calibre barrel , it was designed in the 1960s for the Royal Navy ' s new classes of frigates and destroyers . The new weapon , built by Vickers , was developed by the Royal Armament Research and Development Establishment using the Ordnance , QF 105 mm L13 of the Abbot self - propelled gun as a starting point ( it used electrical primers ) . The outer shell of the gunhouse is built from glass - reinforced plastic ( GRP ) . The new weapon emphasised reliability over a high rate of fire , allowing a switch to a lighter , single barrel mounting and ammunition of a one - piece design . The gun system has a combination of electrical and hydraulic components and the full system penetrates up to three deck levels below the weather deck ; deep magazine , gun control room and power room , gunbay and the gunhouse . The weapon is semi - automatic and can be operated by a smaller crew than its predecessors . With no personnel in the gunhouse , loading is supported by personnel in the gunbay to load the feed ring and in the deep - magazine to pass ammunition to the gunbay . The captain of the gun in the control room ensures continued weapon readiness and the gun controller in the operations room operates the aiming and firing process . The weapon has a rate of fire of about 25 rounds per minute and a range of 12 nm ( 22 km ; 27 . 5 km with the newer High Explosive Extended Range round ) . The first recipient of the new gun and mount , the Mark 8 , was the Iranian frigate " Zaal " in 1971 . The gun entered Royal Navy service in 1973 on the new destroyer " Bristol " . These guns proved to be less reliable than the older 4 . 5 inch Mark V gun ( redesignated Mark 6 gun mounting ) during the Falklands War , being forced to cease fire on several occasions due to faults . The first major modification to the mounting , the Mod 1 , was developed in 1998 in two tranches ; replacing the gunhouse with a reduced radar cross section assembly and replacing the hydraulic loading mechanism with an all electric system . This particular gun has been nicknamed the " Kryten gun " by members of Royal Navy , after the odd shaped head of a robot from the British Sci - fi comedy series " Red Dwarf " . Babcock upgraded 13 guns to Mod 1 standard between 2005 and 2012 . 155 mm variant . The Ministry of Defence investigated a proposal from BAE Systems to " up gun " the 4 . 5 inch to accept the 155 mm ( 6 . 1 inch ) gun barrel and breech from the AS - 90 self - propelled gun . This " 155mm Third Generation Maritime Fire Support " ( 155 TMF ) would introduce a common gun calibre with the British Army and Royal Navy , helping with ammunition logistics , and encouraging joint Army - Navy development of extended range and precision guided shells . A £4m contract has been awarded to develop a prototype , and firing trials were scheduled for 2009 with delivery in 2014 but the project was cancelled in the 2010 SDSR . Future RN warships will have a 127mm gun from either Oto Melara or BAE .