A firearm with a clamping device for attaching a barrel with a barrel extension, where the barrel extension on the one hand, and a seat on the other hand, possibly directly the upper receiver, are connected to one another by means of a clamp and the fixing of the barrel extension on the upper receiver by means of a tension lever that is rotatable about a common axis of rotation running normal to the weapon center plane between two end positions, the ready-to-fire position and the dismantling position, with at least one clamping section and a grip and a cam formed on the tension lever.