The present invention relates to a gun for using compressed gas to propel an arrow. The gun comprises a hollow barrel adapted to accommodate the arrow. The barrel has an open rear end and an open front end. The gun also includes a cylindrical chamber having a front end through which the barrel extends, a closed rear end, and a gas inlet for connecting the cylindrical chamber to a source of compressed gas. The cylindrical chamber is adapted to hermetically enclose the open rear end of the barrel and is axially movable between a non-firing position where the closed rear end of the cylindrical chamber sealingly engages the open rear end of the barrel, and a firing position where the closed rear end of the cylindrical chamber is displaced from the open rear end of the barrel to allow compressed gas to pass into the barrel via the open rear end of the barrel so as to propel the arrow out the open front end of the barrel. The gun has a simple and robust construction, and has a well-balanced behavior during use.