You might be interested in these two cases:
Finally, the government argues that the Gun Control Act, of which section 922(o) is a part, should not be viewed as repealing the National Firearms Act, citing a provision of the Gun Control Act passed in 1968 to that effect. The court in Rock Island Armory rejected the same government argument, observing that "the 1968 Congress cannot bind the Congress of 1986, which decided to ban transfer and possession of machineguns. P.L. 99-308, 100 Stat. 453 (May 19, 1986). Further, a Congressional declaration in 1968 does not solve a constitutional problem which arose in 1986. The ban enacted in 1986 and the government's refusal to accept registrations and tax payments, simply left the registration requirements with no constitutional basis."
In sum, since enactment of 18 U.S.C. sec. 922(o), the Secretary has refused to accept any tax payments to make or transfer a machinegun made after May 19, 1986, to approve any such making or transfer, or to register any such machinegun. As applied to machineguns made and possessed after May 19, 1986, the registration and other requirements of the National Firearms Act, Chapter 53 of the Internal Revenue Code, no longer serve any revenue purpose, and are impliedly repealed or are unconstitutional. Accordingly, Counts l(a) and (b), 2, and 3 of the superseding indictment are DISMISSED
You'll note that FedGov never appealed these decisions, which basically strike down the 1934, 1968, and 1986 GCAs.