totally dependant upon exactly what gun ya are talking about..... the ATF determines what constitutes easy conversion to a full auto in the case of the AK as long as the third axis pin hole is absent then your o.k and no other parts matter, however because of the Drop in auto sear being available for the AR15 the use of a FA bolt carrier is illegal because this is one of the components needed to allow the auto sear to convert the gun to full auto, same for FA FCG parts as these are a the other part of an autosear F/A conversion requiring no machining of the reciever , with an AK ya must have the third axis pin hole and a cut in the right side lower bolt rail in order to make it a full auto so ya can use any original parts ya wish..... that third pin hole is an automatic conviction and all of the original FA parts are not able to convert it to FA without these receiver mods........
Two totally different gun designs so ya got two different laws/regs if ya already have the F-A parts mentioned in an AR 15 then all thats needed is a few seconds to install an auto sear to complete conversion......