The PSM is a Soviet pistol from the late 1970s which has gotten itself quite the fanciful reputation here in the US, thanks to extreme rarity and some imaginative magazine articles. Common lore would have you believe that the PSM and its 5.45x18mm bottlenecked cartridge is capable of astounding feats of armor penetration, and that it was designed specifically for KGB assassins.
The truth is rather more mundane – the PSM was a sidearm for high ranking officers who did not want to deal with carrying a Makarov pistol. Much like the US use of the 1911 and the Colt 1903 back during WW2, general-rank Soviet officers carried sidearms as badges of rank, not as actual combat weapons. To that end, the PSM is extremely thin to make it as unobtrusive as possible. The 5.45x18mm cartridge is basically a centerfire .22 long rifle ballistically. It does offer armor penetration that would be surprising to some, because its metal jacket, mild steel core, and small frontal area are all beneficial in piercing Kevlar. That is a side effect of the design, however, and not an original intent.
Mechanically, the PSM is a simple blowback action, and very similar to the Makarov.
Thanks to Mike Carrick of Arms Heritage magazine for providing the PSM and its ammunition for this video!