The rivet gun has a barrel, a handle, an inflator, a piston assembly and at least one buffer device. The piston assembly has a piston rod and a piston head. The at least one buffer device is mounted in the piston head and has a housing, a valve stem and a buffer spring. The housing has an inserting hole and a side hole communicating with the inserting hole. The inserting hole has a valve opening. The valve stem is movably mounted in the valve opening. A valve part is formed on a lower end of the valve stem to selectively close the valve opening. The buffer spring is mounted around the valve stem. When the piston head moves upward, the buffer spring absorbs some momentum produced by the high pressure gas to mitigate shake caused by the high pressure gas directly hitting the piston head.