I'm not sure, but it sounds like it could be a 7.62x39 with a bullet that slugs to .308" instead of the standard .310".
Most AKs will use a barrel made to .310" to make better use of surplus and East European sourced ammo (Wolf, Brown Bear etc.), while the Mini-30 (IIRC) uses a .308" bore. Though I never understood their reasoning behind the American barrel size if the allure of getting something in 7.62x39 was cheap com block ammo...