You are actually going to have as much of a problem with US Customs as you will have with the ATF. I'm not even positive that you can legally do what you are considering, since, for taxation purposes, it will be considered an import by ATF and US Customs, regardless of how long you have owned it. Being an import, it will have to go through a licensed dealer, or, better yet, a really sharp importer, and a Customs house broker.
It is (or, used to be) pretty easy to return from outside the US with your own shotgun IF, and this was a huge if, you had the little chit (Customs form 4457) properly filled out by a US Customs officer BEFORE you left. You don't. That will be a problem.
I am positive that there are US Customs officers that are professional, helpful, and willing to go the extra mile to see you and your effects happily returning to the US from wherever-but I NEVER met any like that. Not once. They took special joy at Port of Minneapolis in making anyone with a firearm, miserable. PLEASE don't believe you can simply show up at US Customs, after an international flight, with a shotgun and try to make the point that it is your gun, without form 4457 being properly filled out and dated.
Duty and ATF excise tax, both collected by US Customs, will be due on the gun, in the event you successfully bring it to the US.
I wish you good luck. I'd honestly ponder selling it in Europe before attempting what you are considering.
Best,
Ted
