A fastener driving tool including a housing enclosing a cylinder having a lower end provided with a resilient bumper, a return air chamber in communication with the lower end of the cylinder and a piston dimensioned for reciprocation within the cylinder to impact the bumper at an end of the cylinder and having a driver blade depending therefrom for striking fasteners. At least one inlet opening is disposed in the cylinder and in communication with the return air chamber and at least one outlet opening is disposed in the cylinder and spaced from the at least one inlet opening. The at least outlet opening is in communication with the return air chamber and aligned with the piston so that each outlet opening is closed by the piston to seal the cylinder from air loss as the piston impacts the bumper and traps a residual volume of air in the sealed cylinder below the piston to damp impact of the piston upon the bumper.