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. Mk X increased the bore length from Mk IX's 480 inches (40 calibres) to 540 inches (45 calibres), increasing muzzle velocity from 2,600 to 2,700 feet/second.
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.
From 1917 several Mk X guns were deployed ashore on the section of the Belgian coast still held by the Allies, near Nieuport. 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.