Wikipedia
During World War I, the United States secretly developed the Pedersen device attachment for the M1903 Springfield rifle that allowed it to fire a .30 caliber (7.62 mm) pistol type cartridge in semi-automatic mode. This attachment was developed to allow an infantryman to convert "his rifle to a form of submachine gun or automatic rifle" in approximately 15 seconds.
Production of the Pedersen device and modified M1903 rifles started during 1918. However, the war ended before they were sent to Europe. The contract was cancelled on March 1, 1919, after production of 65,000 devices, 1.6 million magazines, 65 million cartridges and 101,775 modified Springfield rifles.
The devices, magazines, ammunition and rifles were subsequently placed in storage, and declared surplus in 1931. To prevent them from falling into the hands of the lawless, nearly all of the stored devices were destroyed by the Army except for a few examples kept by Ordnance Department. Fewer than 100 Pedersen devices escaped ordered destruction to become extremely rare collectors' items.