Legal research? Or planning to use the photos to convert your own?
The request does sound strange. But, if the guy's just looking to make illegal machineguns, all he has to do is use google, or buy the little booklet on line. I doubt that this is for "legal research", though. Legal research involves looking up statutes and cases that address a particular subject. Not photographing someone else's $14,000 machinegun.
The OP apparently sells some CD's or DVD's on various topics. Just wondering if he plans on making a DVD on how to make an M16 and needs pictures?