Better known, as the US Automatic Pistol, Caliber .30, Model of 1918. Yep. Pistol. All part of the deception plan to hide what we were doing from the Germans. Made easier by the fact that it shoots a .30 caliber pistol cartridge, virtually identical to the French .30 Longue, IIRC.
The "automatic pistol" hiding in the body of a bolt-action rifle was the brainchild of John Pedersen, of Remington Arms. Development started before the US entered the war and was a private venture on the part of Remington. Having been following the war, Mr. Pedersen was struck by the inability for troops to effectively fire on the run while crossing "No Mans Land". He started down the path that would lead to such weapons as the Browning Automatic Rifle and Chauchat machine gun. The difference is Pedersen figured that he wasn't going to be able to sell the Army on a whole new rifle design, so he decided to develop a drop-in adaptor that would convert the existing rifle - and at the same time preserve it's hi-powered, long range capabilities.
This led to his gizmo, which replaced the bolt of the standard Springfield with a long bit of kit consisting of a complete firing mechanism and a small "barrel" for the small round. In effect, the "device" was essentially a complete blow-back pistol minus a receiver/grip using the short "barrel" of the device to fit into the longer chamber of the M1903 Springfield. The mechanism was fed by a long 40-round magazine sticking out of the rifle to the top right, and could be reloaded by inserting a new magazine. New sights were provided at the rear of the device. The system did require one modification to the rifle however, a hole had to be cut in the side of the bolt area to allow the ejection of spent rounds. You may have seen some of those - M1903's with an elongated oval cut out of the side rail. We have one in the Arsenal, though it's one of the ones that was reworked during WWII with a regular scant stock and green parkerizing.
In early 1917 the Pedersen device was tested and ready for production. Pedersen took his weapon to show the Army. He first fired several rounds from what looked to be a standard M1903. Then he removed the bolt, inserted his pistol, and fired several magazines at a very high rate of fire. The officers were astounded at the relative simplicity and performance, and immediately clamped a SECRET label on the whole thing. As I noted, snooker the Germans, the Ordnance Department named it The US Automatic Pistol, Caliber .30, Model of 1918. Plans were initiated to start production of modified Springfields, listed as the US Rifle, Cal. .30, Model of M1903, Mark I. The intent was to have 500,000 ready for the 1919 Spring Offensive. The use of the Pedersen Device in the 1919 Spring offensive was to be in conjunction with the full combat introduction of the M1918 Browning Automatic Rifle (BAR). Pedersen's gizmo was also designed to to fit the US Rifle, Model of 1917 (the American Enfield - which was actually used in greater numbers than the Springfield), and the US Rifle, Model of 1916 ( The Remington Mosin Nagant). Neither of those were ever put into production, although samples of both were made.
With the end of the war, production slowed, though it continued to 1920. Technology overtook the Pedersen device (which we'd know a lot more about had the war continued to 1919) and the BAR and Garand in tandem rendered the Pedersen obsolete. After they were declared surplus, they were all heaped in a large pile... and burned. Most of the Pedersen's in private hands are survivors of the fire - and show varying shades of fire damage. Estimates range up and down, but the current best guess is there are about 250 or so survivors in museums and private hands.
