When I was importing from Europe the people at ATF were solid professionals, who never gave me a moments trouble. US Customs, port of Minneapolis, were NOT. I was actually contacted by ATF (oddly, nobody at ATF can import a gun, they regulate those that do) to come to the aid of an immigrant from South Africa, who received some stunningly bad advice from US Customs about getting his Morgan automobile (couldn't help him, there) and his Grandfathers double shotgun into the US. Both were packed with his household items, and both were seized when they got here. At the point I got involved, the shotgun was less than 1 month from mandatory destruction at the hands of US Customs. I got it out for him.
My feelings about US Customs, port of Minneapolis, is that it is a fiefdom of local political appointees, that is really accountable to no-one, and that they interpret the rules as it suits their collective fancy at any given moment-just a hint, if you are the guy legally trying to get a gun past them, good luck with that. I did it for a decade, and they never made it pleasant, and seemed to go out of their way to make it as difficult as possible.
I never tried it, but, I got the vibe that getting a pre 1899 gun past them on the argument that it was old enough to not qualify as a gun would be the supreme test of their indulgence, and was unwilling to even attempt it.
Perhaps US Customs people employed in a less liberal part of the country would be better, or easier to deal with. I wouldn't bet on it.
While you might be tempted to believe a Custom house broker is a lacky of US Customs, my guy was an ace, and on occasion, I witnessed him putting US Customs in it's place. Keep in mind, your package from Europe, containing a post 1899 gun, that mistakenly gets off-loaded at an airport that is not it's final destination, will be seized, period. The customs house broker will get it out, I can promise that you can't.
Been there, done that.

Best,
Ted