Another thing to remember is that just because someone is an expert with the skills and knowledge he uses in his job doesn't necessarily make him an expert in other, even if related, matters. Combat in a war zone is not exactly the same as self defense by a private citizen on a peaceful city street. And a local police officer shouldn't necessarily be expected to be fully familiar with federal law on firearm transfers.
"It is long been a principle of ours that one is no more armed because he has possession of a firearm than he is a musician because he owns a piano. There is no point in having a gun if you are not capable of using it skillfully." -- Jeff Cooper