"pounds" to an actual barrel diameter requires consulting a historical reference. A mixture of designations were in use for land artillery from the First World War (such as the BL 60-pounder gun, RML 2.5 inch Mountain Gun, 4 inch gun, 4.5 inch howitzer) through to the end of World War II (5.5 inch medium gun, 25-pounder gun-howitzer, 17-pounder tank gun), but the majority of naval guns were by caliber. After the end of World War II, field guns were designated by caliber. Types There are many different types of shells. The principal ones include: Armor-piercing shells With the introduction of the first ironclads in the 1850s and 1860s, it became clear that shells had to be designed to effectively pierce the ship armor. A series of British tests in 1863 demonstrated that the way forward lay with high-velocity lighter shells. The first pointed armor-piercing shell was introduced by Major Palliser in 1863. Approved in 1867, Palliser shot and shell was an improvement over the ordinary elongated shot of the time. Palliser shot was made of cast iron, the head being chilled in casting to harden it, using composite molds with a metal, water cooled portion for the head. Britain also deployed Palliser shells in the 1870s–1880s. In the shell, the cavity was slightly larger than in the shot and was filled with 1.5% gunpowder instead of being empty, to provide a small explosive effect after penetrating armor plating. The shell was correspondingly slightly longer than the shot to compensate for the lighter cavity. The powder filling was ignited by the shock of impact and hence did not require a fuze. However, ship armor rapidly improved during the 1880s and 1890s, and it was realised that explosive shells with steel had advantages including better fragmentation and resistance to the stresses of firing. These were cast and forged steel. AP shells containing an explosive filling were initially distinguished from their non-HE counterparts by being called a "shell" as opposed to "shot". By the time of the Second World War, AP shells with a bursting charge were sometimes distinguished by appending the suffix "HE". At the beginning of the war, APHE was common in anti-tank shells of 75 mm caliber and larger due to the similarity with the much larger naval armor piercing shells already in common use. As the war progressed, ordnance design evolved so that the bursting charges in APHE became