Russia uses the AK74 now. It fires a smaller .223ish bullet. 1989 banned the importation of foreign "Assult rifles" so real AKs havnt been around since then..hence the MAK90s and other variants.
Technicaly a machine gun is usually an open bolt bolt gun, but as far as the law goes anything that fires more than once with when the trigger (even a double barrel shotgun with 2 hammers and one trigger) is pulled is a machine gun. Machine guns are under the National Firearms Act of 1930sumthin (NFA) which states they have to be registered and a $200 dollar stamp tax be paid along with some other stuff. In 1986 they banned the manufacuer of new transferable machine guns to civilians. (1969 they did the same with imported machine guns hence the expense of AKs MP5s etc.) So all machineguns registered before 1986 are all thats availible to civilians for the rest of time. That is why they are supremely expensive. So machine guns, silencers, sawed of shotguns, grenade launchers and more can all be owned leagally, (states may have their own ban however) but must be registered in accordance with the NFA rules.
Hope that answeres some questions.