We all know we're not "in the business" when we help out a friend, but it's almost impossible to prove that we didn't get some compensation, however intangible, from it. And remember, once ATF has their hooks in you, you are paying an attorney ten bucks a minute to fight with them. There is a word for people who argue with ATF on their own- felons. If you rob a convenience store and shoot a dozen orphans on the sidewalk when leaving, you're innocent until proven guilty. If you redrill a stripped scope mounting hole on your buddy's rifle so he can take it elk hunting the next day, and don't have an FFL, you're guilty until proven innocent. The ATF has an institutional belief (or at least they used to) that every FFL holder is a criminal that just hasn't been caught yet.