By the outbreak of war in 1914, the majority of navies with submarines wanted a gun as an alternative to the torpedoes, but it was the Royal Navy which sent the largest gun to sea in a Submarine in the M class.The thinking behind such a weapon was made clear by Rear Admiral Submarines "The object was to supplement torpedo attacks against surface ships, which could often elude a torpedo. A 12 inch projectile fired at fairly close range should be difficult to elude. Furthermore, fifty shells could be carried instead of a few torpedoes."

The guns were taken from a Majestic Class pre-dreadnought awaiting the breakers yard. The gun and mounting weighed 129 tons. The size of the mounting actually enhanced the submarines diving qualities - the weight helped to keep her down, whilst it's volume stabilised the boat at periscope depth.