A pneumatic nail gun includes a passage defined through the end cap and the passage communicates with a space between the cylinder in the body of the gun and an inside of the body. The cylinder has an opening which communicates with the space so that when pulling the trigger to eject a nail and then releasing the trigger, the pressurized air is supplied via the passage in the end cap and pushes the piston back to its ready-to-fire position more quickly than the conventional nail gun. The space may also be made in a form of an interior pipe so that the area of the piston can be as big as the inner diameter of the hollow body of the nail gun.

A cap assembly for a nail driving gun includes a cylindrical body mounted detachably on an open end of a gun housing. The cylindrical body includes a first flange that defines an inner space and that is formed with an external thread engaging threadedly the open end of the housing, and a second flange that is reduced from the first flange, that extends into the housing, and that defines a receiving space in fluid communication with the inner space. A nut is disposed in the inner space. A cover with an air outlet is fastened to the nut through a bolt to cover the inner space. A sealing member is movably and sealingly disposed in the housing for sealing the receiving space during a nail impelling operation.

A cover which covers outer peripheries of fastening screws are disposed on an outer periphery of a hammer case. A stopper which detachably positions and fixes the cover to the outer periphery of the hammer case is disposed.

A pneumatic fastening tool assembly that employs an engine having a sliding sleeve arrangement to control the supply of air to and exhaust from the pneumatic engine. The sliding sleeve arrangement eliminates the need for a conventional main valve and thereby reduces the overall weight and length of the pneumatic fastening tool relative to those tools that employ a conventional engine configuration.