Firearm, in particular a carbine, with an upper receiver and a lower receiver, the upper receiver comprising at least one barrel with a barrel axis and a barrel extension that can be firmly connected to the barrel, a locking mechanism, a firing pin mechanism, optionally a gas mechanism, the lower receiver comprising at least one grip, a magazine and a trigger mechanism, the upper receiver and the lower receiver being connected to one another in the assembled, ready-to-fire state, characterized in that, on the housing, especially on the upper receiver a seat is provided or integrally formed, and in that, one hand, the barrel extension and, on the other hand, the seat, possibly directly the upper receiver, are connected to one another by means of a clamp, in that, the barrel extension and the clamp comprise corresponding prismatic guide surfaces (321, 62) running parallel to the barrel axis in the mounted, ready-to-fire state of the weapon, that the barrel extension is fastened on the upper receiver by means of a tension lever, which is rotatable in the upper receiver about an axis of rotation that runs normal to the weapon center plane between two end positions, the ready-to-fire position and the dismantling position, with at least one excentre and an grip and a nose formed on the tension lever, in that, a pin being provided for connecting the clamp and the tension lever along the axis of rotation through at least one clamp hole or excentre hole formed normal to the weapon center plane, and in that, in the ready-to-fire position, the barrel extension rests with a rear stop surface on the seat and is clamped backwards by the nose against the forward barrel direction and that the excentre biases the clamp in the vertical direction, and in that, in the dismantling position, the excentre and the nose lie entirely outside the path of movement of the barrel extension in the forward barrel direction. on the housing, especially on the upper receiver a seat is provided or integrally formed, and in that, one hand, the barrel extension and, on the other hand, the seat, possibly directly the upper receiver, are connected to one another by means of a clamp, in that, the barrel extension and the clamp comprise corresponding prismatic guide surfaces (321, 62) running parallel to the barrel axis in the mounted, ready-to-fire state of the weapon, that the barrel extension is fastened on the upper receiver by means of a tension lever, which is rotatable in the upper receiver about an axis of rotation that runs normal to the weapon center plane between two end positions, the ready-to-fire position and the dismantling position, with at least one excentre and an grip and a nose formed on the tension lever, in that, a pin being provided for connecting the clamp and the tension lever along the axis of rotation through at least one clamp hole or excentre hole formed normal to the weapon center plane, and in that, in the ready-to-fire position, the barrel extension rests with a rear stop surface on the seat and is clamped backwards by the nose against the forward barrel direction and that the excentre biases the clamp in the vertical direction, and in that, in the dismantling position, the excentre and the nose lie entirely outside the path of movement of the barrel extension in the forward barrel direction.