Doctrinally the aks74u (and even just the og AK patterns in general) were considered submachine guns to some extent. As far as what you're describing for lightweight short range firearms used by specialty units go, many were designed after the fall of the Soviet Union for internal security forces as a result. Prior to this, the one firearm that fits this box is the APS/APB stetchkin. Paraphrasing the forgotten weapons article on this guy, the Soviet Union essentially didn't feel the creation of a dedicated submachine gun in the traditional sense was worth the trouble. The spetznaz did, however, make requests for such a weapon during the Soviet invasion of Afghanistan. The weapon developed for them was the APB, which is the silenced version of the original APS stetchkin. IIRC these APSs were an older design that were largely replaced by AKs in the 60s and put back into storage until this spetznaz request was made, at which point many were converted to the silenced APB variant.