I've done it once, from England, using a dealer experienced with shipping to this country. In the case of England, you have to write a letter requesting an export permit, basically saying that the gun will be used for sporting purposes. That's pretty straight forward.
On this end, you have two options: You can either use a licensed importer--in which case you'll pay some fees for the service--or you can use anyone with an FFL. (They're allowed to do imports on an "occasional" basis.) Once you give the licensed importer the specific information on the gun in question, he takes care of everything else. If you use a dealer who's not an importer, then you and the dealer need to fill out a form, send it to ATF, and wait for an approved permit to import. You can actually have the firearm come directly to you, in the latter case. I picked mine up at the local post office, did the yellow sheet with the local dealer who had signed the application, paid his nominal fee, and that was it. Import duty on used guns is minimal, something like 4% of the value, plus a nominal customs fee. I did mine about 10 years ago, and on a gun for which I paid about $1500, it cost me about $80 total in duty, customs fees, and the fee to my local dealer.
