[ This guest post was written by Michael. ]
The Pedersen Automatic, or Remington model 51, found little favor in its life between the Great War and the following depression, though not for its lack of mechanical merit. John Pedersen, also known for the Pedersen device which converted the Springfield 1903 into a semi-automatic .30 carbine whose utility was deprecated after WWI and the development of the M1 carbine; the .276 Pesersen cartridge that the M1 Garand was to be chambered in until logistics of ammunition distribution, unfortunately, cast it aside as well; and his toggle-delayed self-loading rifle requiring lubricated cartridges, which did not pass American army trials against the Garand and was not successfully marketed to the Japanese between the wars.
The model 51 was a pocket pistol chambered in .380, and later .32 ACP, but not of the conventional direct-blowback design. It features a momentum block to retard the action from coming out of battery until pressure is lowered. The momentum block is situated inside the slide, and in its locked position, extends below and engages the shoulder on the frame, disallowing pressure on the breech face from further opening the action.
Upon firing, the recoil moves both the slide and block rearward about 2mm for the slide to gain the necessary initial momentum required for the action, at which point the protruding momentum block comes to a stop against the locking shoulder of the frame, arresting its movement and keeping its face against the chamber, containing the pressures. As the slide continues its rearward travel from the initial impulse, a ramped surface on its interior engages an opposing ramped surface on the momentum block, pulling it upwards and inside the slide, thereby disengaging it form the frame allowing the group to fully recoil and cycle the weapon. The barrel, while fixed, was not pressed into the frame as many of similar design were, but held stationary by the takedown pin, allowing for both the removal of the slide and the captive recoil spring around the barrel.
This atypical system of delay, not fully dependent on the mass of the slide as other designs were, allowed the slide to be more delicate and thinner, making it more comfortably pocketed and shaving a couple ounces off compared to other guns of the same size. Other than the slim profile and weight of the pistol, there were other innovations as well. It features a magazine safety, a grip safety that blocked the sear, and the manual safety locked the grip safety, allowing it to be carried chambered with great confidence. The entire pistol could be detail stripped without the employ of any tools; this of course becomes of great novelty once the operator discovers it instead requires three hands.
In its decline, it was also prototyped in .45ACP and accepted for trials by the navy, but again pushed aside in favor of Remington escalating 1911 production for WWII.
More objectively, the grip shape and angle provide an excellent natural point of aim, followed by a crisp trigger pull, a mild and smooth recoil, and accuracy only limited by the round and the shooter. Though nearly six decades my senior, it still accompanies my 1911 with me when I feel inclined to have a backup.