Depth charge thrower Mk II. Depth charges were introduced into the Royal Navy in 1915 as an anti submarine weapon. They consisted of a steel cylinder filled with 300lbs of TNT. There was a hollow tube through the centre of the cylinder into which was fitted the pistol primer and primer placer gear. The pistol primer was set with the depth at which the charge was required to explode which was operated by the pressure of the water as it sank below the surface. The Mark II depth charge thrower continued in use during the first part of World War II before being replaced by the Mk IV pattern.