The BL 4-inch Mark VIII naval gun was a British medium-velocity wire-wound naval gun introduced in 1908 as an anti-torpedo boat gun in smaller ships whose decks could not support the strain of the heavier and more powerful Mk VII gun.

Mk VIII history

The gun succeeded the QF 4-inch Mk III, whose 25 lb shell had been considered insufficiently powerful for its intended role. The BL Mk VIII fired a 31 lb shell. It armed the following warships : The gun was succeeded in its class from 1911 by the QF 4-inch Mk IV. In World War II many guns were used to arm merchant ships.

Mk XI submarine gun

A Mark XI-variant was adapted to arm the K-class submarines laid down 1915.

Sources