As long as the item in question was not US property that was given away for FREE to an allied nation ... the US Arms industry has been protected from competing with freebie govt giveaway stuff coming back to roost.
eg: all those genuine M-14 parts that came from the US and were given to Israel, may not be welcome back home.
You should really be asking this question on an American gun forum, as we Canucks would not be the most reliable source for information on American rules.