I'm guessing that if any charges are filed, this will look more like a tax case with a little firearms law buggery thrown in, than anything else.
A little back story on the man, and firearms import law -
Bob Adams is/was a dealer in high end collectible guns, mostly 4 figures - low five figures antiques. What I'm guessing is that they are going over every gun he has and everyone he's sold to see if he didn't pay the required import tax(10 - 11 %).
They'll also be looking at them to see if he's stamped them with the proper re-import information. When firearms are reimported into the States, the importer's information has to be stamped on to a permanent place on the gun. The problem with this is that the restamping drops the collectible value of the gun 40-50%, so it is conceivable that a expensive importer might not restamp even though required by law, to boost profits.
This will be a hard case for them if they try it, as there weren't really any good records kept on firearms till the 60's, so they'll be stuck trying to figure out where most of those antiques came from.