It appears to be a German Becker M2 20mm cannon, designed for and first used by the Imperial German Air Service. Germany was a supplier of Nationalist China during the early 30s so it fits.

This particular model is a long barreled model for ground use, and does not have a barrel shroud.

More pics: https://www.google.com/imgres?imgurl=http://www.bevfitchett.us/heavy-machine-guns/images/3036_40_126-oerlikon-great-war.jpg&imgrefurl=http://www.bevfitchett.us/heavy-machine-guns/automatic-cannon-the-mm-guns.html&h=2235&w=1213&tbnid=xOs8bUSKN9CK3M:&docid=OKo2JcUgAQ3LoM&hl=en-us&ei=DaIDV6qdOYramAGLhbmoAQ&tbm=isch&client=safari&ved=0ahUKEwjq14T8svfLAhUKLSYKHYtCDhUQMwg7KBUwFQ