1. IN A RIFLE BARREL, THE COMBINATION OF AN OUTER TUBE CONSISTING OF A FIBERGLASS REINFORCED PLASTIC, AN INNER METAL FLEXIBLE TUBE HAVING A SMOOTH BORE AND SUBSTANTIALLY COEXTENSIVE IN LENGTH WITH SAID OUTER TUBE, SAID TUBES LONGITUDINALLY THEREOF AND HAVING A LONGITUDINAL TWIST CONFORMING TO A DESIRED RIFLING OF SAID BARREL, SAID OUTER TUBE EXERTING THOURGH SAID RISERS A FORCE WHEREBY SAID INNER TUBE IS PRESTRESSED IN COMPRESSION AND THE WALL OF SAID INNER TUBE HAVING A THICKNESS SUCH THAT SAID SMOOTH BORE IS CHANGED TO A RIFLED BORE BY FLEXURE UPON THE APPLICATION OF A PREDETERMINED GAS PRESSURE THEREIN.