Apparently Congress authorized some military contractors to be exempt from the Hughes Amendment ('86 MG ban) :
BillingsGazette.com :: Burns', Tester's views align on gun rights
"Both men also said they favored doing away with a 1986-passed cap on the number of fully automatic weapons available for sale. At that time, Congress limited the number of fully automatic guns - guns that will fire as long as the trigger is pressed - to those in circulation in 1986. Since then, the price of these limited weapons has gone up. However, civilian police forces are exempt from the cap, and recently Congress allowed certain military contractors to be exempt from the 1986 cap, too".
No idea what law they are talking about, but they seem to know of it. Likely an obscure amendment tacked onto a school lunch program or somesuch. The new ATF reform bill that is sitting in Congress apparently wants to expand that exemption:
Search Results - THOMAS (Library of Congress)
(b) Sale or Delivery of Machineguns to Federal Contractors- Section 922(b) of such title is amended by adding at the end the following: `Paragraphs (2) and (4) of this subsection shall not apply to a sale or delivery to comply with a contract between any person and the United States which requires that person to provide national security services for the United States or any training related to the services.'.