Abstract: A firearm is disclosed which includes a gun body having a firing mechanism and a barrel mount movable between a barrel loading position and a firing position. The firing mechanism is maintained in inoperative condition unless the barrel mount is correctly positioned in the firing position. A shoulder stock is releasably mountable on the gun body to permit the firearm to be selectively usable in pistol or rifle form. A plurality of barrel parts are selectively mountable on the gun body and include a first barrel of a length capable of firing baton rounds and gas rounds and a first insert of a similar length adapted to be fitted from only the end adjacent to the barrel mount into the first barrel to provide a smaller caliber barrel. A second barrel of a shorter length than the first barrel is provided and is capable of firing flare rounds.