RML 9 inch 12 ton gun The RML 9 inch guns Mark I - Mark VI were large rifled muzzle - loading guns used as primary armament on smaller British ironclad battleships and secondary armament on larger battleships , and also ashore for coast defence . Design . The rifling was the " Woolwich " pattern of a relatively small number of broad , rounded shallow grooves : there were 6 grooves , increasing from 0 to 1 turn in 45 calibres ( i . e . 405 inches ) . Mark I , introduced in 1865 , incorporated the strong but expensive Armstrong method of a steel A tube surrounded by multiple thin wrought - iron coils which maintained the central A tube under compression , and a forged steel breech - piece . 190 were made . Mark II in 1866 incorporated the modified " Fraser " design . This was an economy measure , intended to reduce the costs incurred in building to the Armstrong design . It incorporated fewer but heavier wrought - iron coils but retained the Armstrong forged breech - piece . Only 26 were made . Mark III in 1866 - 1867 eliminated the Armstrong forged breech piece and hence fully implemented the Fraser economy design . It consisted of only 4 parts : steel A tube , cascabel , B tube and breech coil . 136 were made . Mark IV , introduced 1869 , and V incorporated a thinner steel A tube and 2 breech coils . The explanation for separating the heavy breech coil of Mk III into a coiled breech piece covered by a breech coil was " the difficulty of ensuring the soundness of the interior of a large mass of iron " . Mk VI high - angle gun . In the late 1880s and early 1890s a small number of guns were adapted as high - angle coast defence guns around Britain : known battery locations were Tregantle Down Battery at Plymouth , Verne High Angle Battery at Portland and Steynewood Battery at Bembridge on the Isle of Wight . The idea behind these high - angle guns was that the high elevation gave the shell a steep angle of descent and hence enabled it to penetrate the lightly armoured decks of attacking ships rather than their heavily armoured sides . To increase accuracy the old barrels were relined and given modern " polygroove " rifling : 27 grooves with a twist increasing from 1 turn in 100 calibres to 1 turn in 35 calibres after 49 . 5 inches . These guns fired a special 360 - pound armour - piercing shell to a range of 10 , 500 yards using a propellant charge of 14 lb Cordite Mk I size 7½ , remained in service through World War I and were not declared obsolete until 1922 . Some guns were bored out and relined in 10 - inch calibre . A battery of six such guns is known to have been mounted at Spy Glass Battery on the Rock of Gibraltar , and six guns at Gharghur , Malta . Ammunition . When the gun was first introduced in 1865 , projectiles had several rows of projecting " studs " which engaged with the gun ' s rifling to impart spin . From the mid - late 1870s , spin was imparted by " gas checks " connected to the base of the projectile which engaged the rifling grooves , making studs unnecessary , and hence the shells were designated " studless " . The gun ' s primary projectile was " Palliser " shot or shell , an early armour - piercing projectile for attacking armoured warships . A large " battering charge " of 50 pounds " P " ( pebble ) or 43 pounds " R . L . G . " ( rifle large grain ) gunpowder was used for the Palliser projectile to achieve maximum velocity and hence penetrating capability . Common ( i . e . ordinary explosive ) shells and shrapnel shell s were fired with the standard " full service charge " of 30 pounds R . L . G . gunpowder or 33 pounds P ( pebble ) gunpowder , as for these velocity was not as important .