The 2S7 Pion was identified for the first time in 1975 in the Soviet Army and was called M-1975 by NATO (the 2S4 Tyulpan also received the M-1975 designation), whereas its official designation is SO-203 (2S7). It used a new-design chassis, partly based on T-72 and T-80 design, carrying an externally mounted 2A44 203 mm gun on the hull rear.[​citation needed] The vehicle is self-entrenching and has an overpressure CBRN defense system. It was reported that the 12-metre (39 ft) long gun weighs 14.6 tons and has a service life of 450 rounds.