Forgotten Weapons article on the pistol.

From the article:

The mechanism of the M53 (and also the Model 51) was somewhat unusual, as it had to avoid infringing on Browning’s patents covering features like the single unitized slide and breechblock. Pedersen did this by devising a system in which the slide housed an independent breechblock. Then the pistol fired, the slide and breech would recoil together for a distance of approximately 2mm (1/12th of an inch), at which point the breechblock would lock against a lug in the pistol’s frame. Inertia would force the slide to continue rearward, and after a bit more travel a cammed track in the slide would lift the breechblock up and out of its locked position. The slide and breechblock would then continue the remainder of the way back, ejecting the spent case and then loading a fresh round form the magazine. This system is often misunderstood as being blowback, since the Model 51 is chambered for the .380ACP cartridge and has a fixed barrel (a feature generally associated with blowback pistols).

This mechanism actually had fewer moving parts than Browning’s 1911, had a fixed barrel for better potential accuracy, less felt recoil than the 1911, and weighed less. Had the M53 been present in the original trials for Army pistols in 1907-1910, I suspect it would have beaten out the Browning design. The Model 51 has always been an acknowledged favorite of pistol cognoscenti, and it would be fascinating to how formal US adoption could have done to evolve the M53.