Abstract: A method for converting mortar shells into shells for use in smooth-bore gun barrels having an interior surface. The mortar shells include a warhead having a front end, a rear end, a tail fin, and a propellant charge. A guide casing is attached to at least the rear end of the warhead. The guide casing includes an outside surface. The guide casing supports at least the rear end of the warhead and is adapted for use with the smooth-bore gun barrel that the converted shells are to be fired from. A bourrelet is attached to the outside surface of the guide casing for sealing between the converted shells and the interior surface of the smooth-bore gun barrel that the converted shells are to be fired from.