The Gepard PDW (Russian for Cheetah) is an experimental weapon, based on the AKS-74U. It was designed by Gennady Sitov and Aleksandr Shevchenko in 1995-1997 and suggested for law enforcement agencies but never put into service. The most interesting feature is that the Gepard can shoot no less than five different calibers without any swapping. With swapping out the bolt and barrel, the number of available calibers increases to fifteen.
Depending on the caliber, the weapon fires by blowback, delayed blowback, or gas operation; for most caliber changes, only a change in the recoil springs, bolt, and barrel is necessary.
I wanna shoot one of these. This looks fun as hell.