Officially designated the “Automatic Pistol, Caliber .30, Model of 1918- Mark I”, the Pedersen Device consisted of a semi-automatic bolt detachable magazine designed to replace the standard bolt in a modified Model 1903 Rifle and convert it into a semi-automatic weapon. The device fired a low velocity pistol sized cartridge designated the “Caliber .30 Automatic Ball Pistol” and had a 40-round magazine. The Pedersen Device was a secret weapon intended to provide U.S. Infantry with a close range semi-automatic rifle for assaults on enemy trenches and still retain the advantages of a high powered bolt action rifle for long range combat. The Army planned to introduce the Pedersen Device in the offensive against the German Army in the spring of 1919. Remington Arms manufactured 101,775 Pedersen Devices. When WWI ended in November 1918, the Pedersen Devices were placed in storage. In 1931 the security classification was removed from the Pedersen Device and all of the devices were ordered to be destroyed. The destruction was nearly complete; fewer than 100 Pedersen Devices are believed to exist in collections and museums today. The Model 1903 Mark I rifle was manufactured by Springfield Armory in 1918 and identified by special receiver markings and the oval ejection port in the left side of the receiver. Original Model 1903 Mark I Rifles had a special receiver, stock, magazine cut-off, cut-off spindle, trigger and sear. After the Pedersen Devices were destroyed, the Mark I rifles had the special components removed and were converted to standard Model 1903 configuration.

There is a very good VIDEO to describe the working of this device

    Early commercial headstamp