 The 3"/50 caliber gun Marks 17 and 18 was first used as a submarine deck gun on R-class submarines launched in 1918-1919.At the time it was an improvement on the earlier 3"/23 caliber gun.After using larger guns on many other submarines, the 3"/50 caliber gun Mark 21 was specified as the standard deck gun on the "Porpoise"- through s launched in 1935-1942.