I'd never heard of it until today. Kind of reminds me of the 5.7 round.
Small-caliber pistol ammunition
The last development, which could be considered as a “special purpose”, took the place during 1970 – 1972 timeframe, when the KGB requested the development of an “easily concealable pocket pistol, with flat shape and not thicker than a matchbox (17mm)”. This pistol was intended as a concealed carry weapon for security personnel, as well as self-defense weapon for top ranked officials. After a short research designers rejected available western “pocket-gun” ammunition, such as 6.35x16SR and 7.65x17SR, as entirely unsatisfactory, and rapidly developed a small round, which looked more or less like scaled down 7.62x39 M43 cartridge. The 5.45x18 MPTs cartridge had a rimless bottlenecked case, made of brass, with pointed jacketed bullet. The terminal performance of this tiny cartridge is rather unspectacular, but it has a reputation for penetrating soft body armor at shorter distances. However, it is by no means an “armor piercing” ammunition, and the latter attempts to use it for larger PDW-type weapons such as full-size OTs-23 “Drotik” automatic pistol, turned into a predictable failure. Still, the extremely compact and quite reliable 5.45x18 PSM pistol is widely used as a self-defense weapon for Army and MVD generals, as well as by various law enforcement operatives, working under cover.
cartridge metric designation bullet weight muzzle velocity 7N7 / 5.45mm MPTs 5.45x17 2.5 g / 39 grain 320 m/s / 1050 fps