I want to start off by stating that I HAVE NOT exported anything. I have been importing my own firearms however and it really isn't that difficult. The hardest part is getting good information and the agencies that are in charge often don't even know their own rules. On top of that there is a lot of bad information that is given by people trying to help. I may just do that now (but I don't think I am). I'm mostly writing this as I hate to see people not do things because they think it's too difficult. I hear that often in regards to importing and it really isn't.
The State Dept. covers ITAR and the firearms in question are not covered by ITAR.
ATF does not handle exports. They have always been great to deal with however when importing and once called/emailed me a half dozen times when approving a form 6 to get additional info and ask questions rather than denying it like many other agencies would have done. Nothing but good to say about them.
I believe most items people (on this forum) would want to export would be covered under ECCN 0A501 or 0A502. Both under the Department of Commerce, Bureau of Industry and Security. It appears to me that they would need and export permit and it looks like anybody can apply for one for free under the online SNAP-R account system.
The word antique is NOT standardized. Not in this country and certainly not internationally. For firearms export purposes it appears to be pre-1890 not pre-1899 like it is for import. If the pistols that Montenegrin wants to export are pre-1890 then they fall under EAR99 and do not appear to require an export permit.
Shipping is likely going to be your biggest hurdle and certainly the most costly. They will most likely have to go air freight and that isn't cheap.
I have no idea what the EU import process is but you mentioned you are familiar with it. If it's allowed the easiest way is probably to fly to the US for a vacation and fly back with them, declaring them (with the previously acquired paperwork) upon your arrival to your home country. This may not be allowed but it would likely be only a bit more money than the shipping and you get a trip out of it.
I don't have any more info on this than what I posted and it may not be 100% correct, but hopefully it gets you on the right path.
