In 1900, John Moses Browning filed a patent for a recoil-powered automatic gun. Browning did not work on the gun again until 1910, when he built a water-cooled prototype of the 1900 design. Although the gun worked well, Browning improved the design slightly. Browning replaced side ejection with bottom ejection, added a buffer for smoother operation, replaced the hammer with a two piece firing pin, and some other minor improvements. The basic design of the gun was still the 1900 design.