all firearms are legal.
the difference, is that some firearms require certain permits to possess under the 1986 Hughes amendment to the NFA.
for possession of a machine gun, you either need a transferrable (any registered MG pre May 1986, extremely rare, and this in good condition are literally worth more than most houses), or an FFL-31-SOT, which allows you to manufacture "dealer samples".
Suppressors (AKA Silencers), and Short Barrel Rifles/Shotguns are now fully transferrable under a form 4473; due to the recent Texas ruling against the NFA.
the requirement for Form 4473s; however, are likely to change due to the semi recent elimination of most of the tax stamp requirements for transferring NFA items; which was only allowed (per the supreme court) because congress has the constitutional power to enact and enforce taxes, but they do not have the power to regulate firearm circulation. the NFA is on its last legs. I will celebrate the day it is finally killed. The victims of the ATF's enforcement over the years deserve renumeration... or at the very least, an apology.