I've never exported firearms but I do have some experience with exporting 'Significant Military Equipment' and will do my best to assist (please note that I am not advising you here, just providing some general information). ITAR Exemption § 123.17.(c) allows the export of firearms to take place without the requirement for a DOS license provided the below conditions are met. You
will need to physically meet with Customs prior to departure (as discussed above). If I were you, I would call ahead of the flight to the Customs office located at Terminal E in Logan and ask to speak to a Customs supervisor- explain to him/her what you are looking to do and give them a rough idea of when you will be bringing the firearm down to them to be signed off (see if you can have this done even on a day prior to your flight if possible to avoid delays)- this will enhance their comfort level as well as your own. Customs does see this type of situation very often, just make sure the weapon's are properly stored/locked up.
Just an added note,you may also be required to file what's known as an electronic SED (Shipper's Export Declaration), or AES (Automated Export System), with the Census Bureau prior to departure in order to obtain an ITN (Internal Transaction Number), this would have to be made available to Customs prior to departure. This is required for all ITAR-controlled items but you can confirm with the Customs official whether this is required (there are some AES exemptions, I just don't know any for firearms off the top of my head and it's unlikely one exists), if you need more information PM me and I'll try to get back to you.
§ 123.17 -- Exports of firearms and ammunition.
(c) District Directors of Customs shall permit U.S. persons to export temporarily from the United States without a license not more than three nonautomatic firearms in Category I(a) of § 121.1 of this subchapter and not more than 1,000 cartridges therefor, provided that:
(1) A declaration by the U.S. person and an inspection by a customs officer is made;
(2) The firearms and accompanying ammunition must be with the U.S. person's baggage or effects, whether accompanied or unaccompanied (but not mailed); and
(3) They must be for that person's exclusive use and not for reexport or other transfer of ownership. The foregoing exemption is not applicable to a crew-member of a vessel or aircraft unless the crew-member declares the firearms to a Customs officer upon each departure from the United States, and declares that it is his or her intention to return the article(s) on each return to the United States. It is also not applicable to the personnel referred to in § 123.18.
As another added note, depending on the total value of what you are bringing, another thought may be to obtain a carnet to relieve any foreign duties or taxes- depending on where you are going and how the foreign Customs operate a
carnet may be a good choice. It functions as a 'passport' for your equipment and relieves any taxes/duties associated with your equipment for foreign Customs. It is not a requirement, but sometimes can be very helpful in getting through Customs without a hassle, the important part to remember is that the carnet
must be signed off both by US Customs and the foreign Customs upon departure
and return, otherwise S. African Customs may initiate a claim for unpaid duties.