The British 15-inch 42 calibre Mk1 gun was the most widely used heavy gun of the twentieth century. It armed twenty-two battleships, battlecruisers, large cruisers and monitors of the Royal Navy. It took part in over eighty naval engagements spanning both World Wars, including famous battles such as Jutland, Cape Matapan and the Denmark Strait.In this new history, Chris Baker looks beyond the wars, campaigns and ships, to analyse in depth, for the first time, the part played by the gun itself in shaping these events. He uses the records of the Ordnance Board to explore the origins and design of the gun, and the difficulties which were encountered in manufacturing it.In addition to analysis of operational use of the gun in the naval battles and bombardments, this book examines the infrastructure which sustained the guns in war and peace. This ranged from shore bases, ammunition supply, and refurbishment and repair arrangements to target and battle practice at sea. Making extensive use of the original gun log records, he examines the deployment and distribution of all 188 guns, and the policies which governed their use.The narrative also explores the extensive use of these guns for trials and experimentation from 1913 to the 1960s, including the prolonged search for improved shell performance. There are chapters covering the decisions to use the guns ashore for coastal artillery, both as the main armament of the Singapore fortress, and to control the Dover Strait in World War Two. The improvements made to the gun performance in the 1930s are described.The book concludes with a description of the fate of the guns during and after World War Two, and the story of how the final two guns in existence came to be saved by the Imperial War Museum in the 1960s.This first comprehensive look at the origins, manufacture, management, use and demise of the 15-inch gun is extensively illustrated, and has detailed statistical analysis of all the guns, and the operations they took part in.