The secret weapon of World War II never used 🤫 

The Pedersen device was an experimental weapon attachment for the M1903 Springfield bolt action rifle that allowed it to fire a .30 caliber (7.62 mm) pistol-type cartridge in semi-automatic mode.[2][3] The attachment was developed to allow an infantryman to convert "their rifle to a form of submachine gun or automatic rifle" in approximately 15 seconds.[2][3]

Production of the Pedersen device and modified M1903 rifles started in 1918.[3] However, World War I ended before they could be fielded.[2][3] 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.[2][3][4]

The devices, magazines, ammunition and rifles were subsequently placed in storage,[2] and declared surplus in 1931.[4] When the United States Army decided they did not want to pay the cost of storing the devices, nearly all of the stored devices were destroyed except for a few examples kept by the Ordnance Department.[1] Fewer than 100 Pedersen devices escaped ordered destruction to become extremely rare collectors' items.[1]