In all likelihood 18 USC sec. 922(o), the ban on civilian possession of machine guns registered after the law took effect, or never registered, precludes an Amnesty (as provided for under existing law) for machine guns. You could register it, and comply with the NFA, but you would still be in violation of sec. 922(o), because the gun would have been registered after the law took effect. The penalties are the same under either law. One could register all other categories of NFA guns at an Amnesty. Congress could also pass a law providing for an Amnesty, and override 922(o) in that manner.
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The definition of “machinegun” in the NFA (26 USC sec. 5845(b)) includes parts to convert a gun into a machine gun. Note that conversion parts are not included in the definition of “firearm” under the Gun Control Act, one of the few things I know of that is a firearm under the NFA, but not the GCA. Thus the purchaser of a conversion part from an FFL need not do a 4473 form, unlike other NFA weapons. Of course the host gun, if purchased from an FFL, will require the 4473. This reading of the law is based on numerous statements from ATF, and the definition of “firearm” under the GCA, which requires it be able to expel a shot. However, at least one very slow judge has decided that somehow the definition of “firearm” in the GCA “incorporates” the definition of “machine gun” under the GCA (even though the law doesn’t say that) and that a machine gun conversion part is a “firearm” under the GCA as well as the NFA. I think the judge is clearly wrong, even ATF reads the law better than that, but the point is to be careful. The case is U.S. v. Hunter, 843 F.Supp 235 (E.D. Mich. 1994), and see also the same judge’s second opinion in the same case, at 863 F.Supp. 462 (E.D. Mich. 1994). These parts are called registered sears, as well as other parts or sets of parts to convert a gun into a machine gun.