The Mk X continued the trend of lengthening gun barrels as far as new construction methods would permit, in order to allow more cordite propellant to be used to attain higher projectile velocities. The Mk X increased the bore length from the Mk IX's 480 to 540 in (12 to 14 metres), increasing muzzle velocity from 2,600 to 2,700 ft/s (790 to 820 m/s).
Subsequent British attempts to further increase the power of 12-inch guns led to failure with the 50-calibre Mk XI and Mk XII guns; the Mk X was the last successful 12-inch British gun.
Naval use
Mk X guns were mounted in the following ships which served throughout World War I:
From 1917 several Mk X guns were deployed ashore on the section of the Belgian coast still held by the Allies, near Nieuwpoort. They were part of the "Royal Naval Siege Guns" under the command of Admiral Sir Reginald Bacon, and were used for attacking German heavy gun batteries.
^Mark X = Mark 10. Succeeding versions were Mark XI (11) and XII (12). Britain used Roman numerals to denote Marks (models) of ordnance until after World War II
^2700 ft/second : As quoted in "Range Tables for His Majesty's Fleet, 1910. February, 1911"; with 260 lb cordite MD size 45 propellant : Treatise on Ammunition, 1915
^22,860 metres quoted at : http://navalhistory.flixco.info/H/76624x53535/8330/a0.htm This is understood to be the maximum possible range at 45° elevation, which was possible for siege mountings on land but not for naval mounts
Bibliography
"Range Tables for His Majesty's Fleet, 1910. February, 1911"