BL 12-inch Mk X naval gun

The BL 12 inch Gun Mark X was a British 45-calibre naval gun which was mounted as primary armament on battleships and battlecruisers from 1906. It first appeared on HMS Dreadnought (1906).

History
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, increasing muzzle velocity from 2600 to 2700 ft/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:
 * Lord Nelson-class battleships, laid down 1905, completed 1908
 * Battleship HMS Dreadnought (1906), laid down 1905, completed 1906
 * Invincible-class battlecruisers, laid down 1906, completed 1908–1909
 * Bellerophon-class battleships, laid down 1906, completed 1909
 * Indefatigable-class battlecruisers, laid down 1909, completed 1911–1913

World War I use ashore
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.

Weapons of comparable role, performance and era

 * 305mm/45 Modèle 1906 gun French equivalent
 * EOC 12 inch /45 naval gun Elswick Ordnance Company equivalent
 * 12"/45 caliber Mark 5 gun US equivalent