[Olle]

You are allowed to send $100 worth of gun parts without any paperwork, more than that and you need an export license. You can actually get a temporary license at no charge for non-commercial shipments, so that might be an option as well. I checked this a while back (regarding Garand stocks), and this is the reply I received from DDTC:

Yes, virtually all firearms parts are on the U.S. Munitions List (USML) and are controlled by the International Traffic in Arms Regulations (ITAR). While most exports of firearms parts require licensing, there is an exemption in ITAR Part 123.17(a)(1) that might apply to your situation:

§123.17 Exports of firearms, ammunition, and personal protective gear.
(a) Port Directors of U.S. Customs and Border Protection shall permit the export without a license of:
(1) Parts and components for USML Category I(a) firearms, except barrels, cylinders, receivers (frames), or complete breech mechanisms, when the total value does not exceed $100 wholesale in any transaction
If this exemption won’t work for you, there is one other potential avenue. Ordinarily, only U.S. entities registered with the State Department may apply for a license to export a defense article, and registration carries the hefty price tag of $2,250 per year. However, in one-time cases involving a personal rather than a business transaction, our Directorate can issue a “one-time exception” license to a U.S. person (citizen or legal permanent resident). I have attached instructions and a form for this procedure in case it fits your circumstances better than the 123.17(a)(1) exemption.

Note that there is an exception for barrels, so these can obviously not be shipped under the $100 rule. I don't know exactly what they mean by "complete breech mechanism" (assembled and functional receiver?), but a stripped breech block should be ok.

And the follow-up regarding the fee:

There is no fee for a one-time exception license. You should expect processing to take about 4 weeks from the time the DDTC receives a complete application.

You may want to contact the DDTC response team ( http://pmddtc.state.gov/response_team/index.html ) and get it straight from the horses mouth. I got a very quick reply and they will hopefully guide you in the right direction.