Possibly the answer to your question is to be found in Alcohol, Tobacco and Firearms Pamphlet 5300.4, Federal Firearms Reference Guide, available here. I'd try to find the answer myself but the document is a huge PDF file that utterly overwhelms my dial-up connection. The National Firearms Act mainly addresses the regulation of importers, manufacturers, dealers and collectors of NFA weapons; how ones in the possession of private military corporations are addressed is an interesting question.