BL 13.5-inch Mk V naval gun

The BL 13.5 inch Mk V gun was a British heavy naval gun, introduced in 1912 as the main armament for the new super dreadnought battleships of the The calibre was 13.5 inches (343 mm) and the barrels were 45 calibres long i. e. 607.5 inches (15.43 m). The guns were greatly superior to the unrelated earlier 13.5 inch (30 calibre) Mk I to Mk IV guns used on the , and es completed between 1888 and 1896.

The gun was developed in response to the relative failure of the British high velocity 12 inch Mk XI and XII guns. Unlike Germany, which developed and deployed successful high velocity 12 inch guns, Britain in this case switched to guns firing larger and heavier shells at lower velocity which could achieve similar performance in range and armour penetration but could deliver a heavier explosive charge on impact.

The gun was wire wound and weighed approximately 168,000 lb (76 tonnes) (excluding the breech), and in its original form fired a 1,250 lb (567 kg) armour piercing capped (APC) or high explosive (HE) round a distance of 23,800 yards (21,800 m) at a 20 degree elevation.

Due to the excellent characteristics of the gun, it was decided to increase the weight of shell to 1,400 lb (635 kg), with an increased firing charge to achieve about the same range. The gun firing the lighter shell was designated Mark V(L) (for "light") by the Royal Navy, and the 1,400 lb version Mark V(H) (for "heavy"). In 1940 these guns were issued to the Royal Marine Siege Regiment at Dover in Kent to bombard German batteries and shipping in the Calais area. They could be stored in railway tunnels when not in use to protect them from attack.

A 13.5/8 inch hypervelocity gun (a type of very large calibre artillery) for stratospheric experiments was developed and deployed near St Margaret's in Kent. The weapon was a 13.5 inch gun Mark V lined down to 8 inches; the liner projected several feet beyond the 13.5 inch barrel. The concept was suggested by F. A. Lindemann, Winston Churchill's scientific advisor. Due to its deployment near the heavy cross Channel guns and manning by the Royal Marine Siege Regiment, it is often erroneously assumed to have been intended as a cross Channel gun. It was initially named Wilfred, but this was soon changed to Bruce, after Admiral Sir Bruce Fraser. The projectiles were custom made with external rifling to match the gun's rifling, with tighter tolerances than normal; this resulted in the need for a screwdriver type tool to ram the projectiles. The rate of fire was very low as a result; but this was not a major concern in an experimental piece. Both high explosive and high velocity shells were made for the gun; the high velocity shell was a smoke shell, intended to burst at high altitude. Observations of the smoke were used to study conditions in the stratosphere. The gun was first test fired in June 1942 at the Isle of Grain, also in Kent. After further experimental firings, the weapon was taken out of service in February 1945.