The D-20 gun-howitzer was developed shortly after the end of WW2 by the F. Petrov Design Bureau; it was intended to replace the pre-war 152 mm ML-20 howitzer (M1937). The barrel length of the gun is 25 calibers. The D-20 was the first 152-mm cannon system to incorporate a semiautomatic vertical-sliding-wedge breech block. The gun was also used for the self-propelled 2S3 Akatsiya. The D-20 is in operation in at least 13 countries and has been license manufactured in the People's Republic of China as the Type 66 (or the improved version Type 66-1). Its self-propelled version known as Type 83 was first introduced in the mid-1980s.