The SKS is only semi-automatic, and, in its unaltered state, has a fixed 10-round magazine. You are right in that the SKS and AKM share a common caliber (although later Kalashnikovs, such as AKM-74, fired the 5.45x39) and this is precisely why their capable range is actually relatively short, along the lines of a 30-30. An un-altered SKS was not illegal under the FAWB. Without the bayonet mount (which many were made without), a SKS, in the original state, would actually lack any of the so called "assault weapon" features. What you identify as a flash suppressor would be little more than any other muzzle brake.
Simply put, it isn't an 'assault weapon' as some call it.