A gas filled medium tension circuit breaker includes an insulating case with a fixed first arcing contact connected to a first circuit breaker terminal. The circuit breaker includes moving equipment formed of a slidable first tube axially displaceable by a drive rod connected to one end of the slidable first tube. The slidable first tube is connected by contacts to a second circuit breaker terminal. The first tube and a fixed second tube define a blast volume. The second tube is extended by a blast nozzle. A second arcing contact is defined by a third tube which slides inside a small diameter portion of the slidable first tube, with that small diameter portion defining with the second tube the blast volume. A larger diameter second portion of the slidable first tube carries the blast nozzle. The two portions of the slidable first tube interconnect radially by a substantially annular portion having through holes and a shoulder. The annular portion constitutes a blast piston which moves axially within the blast volume. The third tube includes a radial flange proximate to the fixed first arcing contact. A compression coil spring has one end bearing against the flange. A second end bears against the shoulder. The drive rod coil spring is compressed when the second movable tube is shifted towards the fixed first arcing contact to cause the arcing contacts to engage end-to-end in a closed position.