Keep in mind thatyou will have different barrel twist on most weapons and then the idea of campatibility and saving money is far away.
Actually not the Czechs are doing such a powerfull round. (if so please tell me which one). I know they necked down a 7.62 Tokarev to 5.6mm and it was a trial for a sub machine gun called VOB, think it was in the last 5-10 years.
The Russiand made already in WWII and API round for the 7.62 Tokarev (then actually for the PPSh-41 other SMGs) and when it comes to vests we have to remember the THV cartridges made in South Africa.
THV = Tres Haute Vitesse - a French development that is basically a very light brass projectile at very high speed which is know as "cop killer" in public (damn TV).
The South Africans used Chinese copper plated steel cases and loaded them with 3 different projectile designs (minor adjustments).
RSA made them also in 5.56x45, 7.62x39, 7.62x51.
In RSA it was called the "MONAD" since they did not use the French name.
Here an image of the 5.56x45:
http://img189.exs.cx/img189/6954/556x45monad6id.jpg