Various embodiments of the present disclosure provide a gas-powered fastener-driving tool. In one embodiment, the tool includes a combustion or precombustion chamber a valve comprising a body that is mobile within the combustion or precombustion chamber between a first closed position and a second open position. The body defines two opposing orifices at longitudinal ends of the body. The orifices form part of a bore defined through the body that enables gas to pass through the valve. This makes it possible to limit the pressure drops as gas is being admitted to or exhausted from the chamber, thus making it possible to optimize the duration of the firing cycle of the tool.