Harlequin: You are thinking of the 9x39 SP5 and SP6 cartridges. It is roughly a 7.62x39mm case necked up to accept 9mm projectiles. The issue ammo tosses a 250gr spitzer at subsonic velocities, putting it in the same class as J.D. Jones' Whisper series of cartridges. There are a variety of weapons chambered for the 9x39mm, ranging from bullpups, compact assault rifles, and suppressed sniper rifles.
You can find some of the 9x39mm weapons at the Tula homepage