A system for use with firearms which permits the ready exchange of barrels on a single action and stock assembly. The barrels may be quickly and easily exchanged under field conditions using a single simple tool and without removing the action and receiver from the firearm stock. Barrels of different calibers may be used on a single firearm leading to greater versatility. The system relies on a pinch lug or clamp attached to the receiver which will draw all barrels onto the receiver and into invariable coaxial alignment with the receiver and action thus insuring the bullet will always have the same trajectory.