There is another option, called a Private Bill, which is a Federal piece of Legislation that is used for the exact limits of what it is said to be. In other words, It is a Federal Law that is passed to apply to ONE specific person/action/event and is not considered precedent under a Court but is accepted as Law. This is often done with regards to people who get caught in a federal law trap where in they are NOT breaking the law, but the law was written in such a way as to make them criminals. Hard to explain but if you Google Private Bill legislation you can find some of them. These are used to add people to lists of benefactors of Laws that somehow left them off by mistake, to clear people of infractions that they own a house in a land that becomes national park/monument and need to repair a road that is on an easemant that is not part of the enabling Legislation.
But I know of two cases of WW2 war trophies being vetted in as registered weapons based on either lack of knowledge about the Amnesty or what the weapon was. One was a gun brought home by a soldier who became a missionary and was out of the country for years at a time, at his passing, his grandson went to his state's senator and got a private bill passed that papered the gun by saying his granddad being out of the country had no knowledge of the illegality of the weapon. The other was a gun brought back and the owner died in the fifties and his widow just kept the hubby's footlocker locked up in the attic until she passed. Again the owner's Heir was able to convince his State senator that there was no intent and no possible way for the Widow to know what was in the footlocker.
"A private bill provides benefits to specified individuals (including corporate bodies). Individuals sometimes request relief through private legislation when administrative or legal remedies are exhausted. Many private bills deal with immigration–granting citizenship or permanent residency. Private bills may also be introduced for individuals who have claims against the government, veterans benefits claims, claims for military decorations, or taxation problems. The title of a private bill usually begins with the phrase, "For the relief of. . . ." If a private bill is passed in identical form by both houses of Congress and is signed by the President, it becomes a private law." This is from the US senates own webpage.
A side piece. As Private Bills are granted for one or very few peoples benefit and CAN often be of or about contentious issues, very rarely are they debated or challenged. In the Senate, they are regarded as a Gentlemans agreement, the Senator is in fact backing the legislation with their own personal imprateur or reputation and thus they almost always pass without objection. They are not rare nor are they out of the ordinary to have such bills covering such things. I know of another Private Bill that allowed a family that inherited a Plains Indian headdress to actually possess it in the family store as otherwise it was a federal offense due the the headdresss containing Eagle feathers.
If you find what can be considered a good find war trophy, approaching a Law Maker with the desire for a private bill registration might very well be your best bet. Some will not touch it. Some are more than willing. How is usually written is that the law does not mention the firearm at all, but instead declares for the relief of john doe, heir of john henry doe. that due to such issues as we see fit, the constraints of law HR XXXX are singularly waived for the period of X to Y, . signed Pres.
what this does is allow the named party to go to the ATF and say, I want to register this as a amnesty weapon, and the law of the USA says I can.
Here is a link to just a list of PB's passed in 1993 just to show the form
http://thomas.loc.gov/cgi-bin/bdquery/?&Db=d103&querybd=@FIELD(FLD001+@4(Private+legislation))
you know when you here that "by a special Act of Congress, John Doe was granted rights to his families Land." or "by a special Act of Congress, John Doe was granted Citizenship to the USA for his service to america" those are private bill legislations.
I do not have a count, but at some time, I was pretty sure there were MANY hundreds in a year that went thru and they were usually bulk packed together to pass without objections.