BL 6 inch gun Mk II – VI The BL 6 inch guns Marks II , III , IV and VI were the second and subsequent generations of British 6 - inch breechloading naval guns , designed by the Royal Gun Factory following the first 6 - inch breechloader , the relatively unsuccessful BL 6 inch 80 pounder gun designed by Elswick Ordnance . They were originally designed to use the old gunpowder propellants . They were superseded on new warships by the QF 6 inch gun from 1891 . Development history . These were Royal Gun Factory designs , although they were also manufactured by Elswick Ordnance . Mark II . Mk II followed the early weakly made and less powerful Mark I 80 - pounder and introduced a 100 - pound projectile , which became standard for British 6 - inch guns until 1930 . It consisted of a much thicker steel barrel with wrought - iron jackets shrunk over it and as originally introduced weighed 81 cwt ( 9072 pounds ) . The gun proved to be too weakly constructed , and 5 steel chase hoops were added to strengthen it and the gun was shorted by 12 inches to rebalance it , resulting in a bore length of 144 inches ( 24 calibres ) and final weight of 89 cwt ( 9968 pounds ) , or 4½ tons . These guns were relegated to non - firing drill use following a burst gun incident on HMS " Cordelia " in June 1891 . Marks III , IV , VI . Mark III finally introduced an all - steel construction , with a steel barrel and steel breech - piece and hoops shrunk over it , weighing 89 cwt ( 4½ tons ) . However , as originally introduced Mk III was still limited to weak charges and low muzzle velocity , and most guns were strengthened by being chase - hooped to allow a full powder charge of 48 lb gunpowder and muzzle velocity of 1 , 960 feet per second . This brought the gun weight up to 100 cwt ( 5 tons ) . Mk IV incorporated the improvements to Mk III . Mk VI differed from Mk IV only in having slightly simplified construction . Marks III , IV and VI became the most commonly deployed versions , and their widespread adoption would indicate they were considered successful . Marks III , IV and VI were interchangeable and had the same performance . They are generally referred to as " 6 - in 5 - ton B . L . R . " in contemporaneous publications such as Brassey ' s Naval Annual . Guns equipped the following British warships : QFC conversion . From 1895 many ships ' guns were converted to QF to use the same brass cartridge case and charge as the modern QF 6 inch guns . They were designated QFC for " QF Converted " , and the new Mark designation began at I over the old gun Mark e . g . I / IV was the first version of Mk IV gun converted to QFC , II / VI was the second version of Mk VI gun converted . Coast defence gun . Mk IV and VI guns were widely used in coast defence around the British Empire , both on hydro - pneumatic disappearing mountings and Vavasseur slides ( inclined slides that absorbed recoil ) . A small number of Mk IV and VI guns had their old 3 - motion breeches replaced by modern single - motion types and the chamber lengthened to accept a more powerful cartridge , and became the BLC ( breech loading converted ) coast defence gun in 1902 . They attained a maximum range of using a 15 oz cordite cartridge . They were replaced by the modern Mk VII as they became available , and were declared obsolete in 1922 . < br style = " clear : left ; " > BLC Siege gun . Mk IV and VI BLC guns were also fitted out with wagons in 1902 to allow them to be transported as semi - mobile siege guns - the gun and siege platform were transported as separate loads , the siege platform was assembled at the firing site and the gun mounted on it . When World War I broke out in 1914 , 2 batteries of these BLC siege guns were equipped with primitive wheeled gun carriages with traction engine wheels and sent to France as heavy field guns . They were towed by steam traction engines . They had limited recoil buffers and required chocks in front and behind the wheels when firing . These guns had a maximum range of 14 , 200 yards . They were soon replaced in action as guns in 1915 by the more modern 6 inch Mk VII and were then converted into 8 - inch howitzers . World War I conversion to 8 inch howitzer . Britain was desperately short of heavy field artillery at the beginning of World War I , and in 1915 old BL 6 - inch guns were bored out and shortened to produce BL 8 inch howitzers as follows : < br style = " clear : left ; " > Mk V . Mk V was a longer ( 30 - calibres , 183 . 5 inch bore ) unrelated Elswick Ordnance export gun .