It doesn't apply to all situations, but there will assuredly be some. Cop shows up at your house because of a medical emergency, while they help you he notices clearly short rifle on your table that you were cleaning. He may not arrest you, or even call the feds right there, but sure as hell it's in his report as a CYA. Now he (Or another anti gun cop who hears about it) calls the feds and dimes you out, maybe giving them the report with all info they need to get you. The feds will be on a tear trying to enforce the piss out of this as an example in those states trying to buck their power, and some people will pay the price for believing that state laws actually protect them in any way, just like those poor bastards from kansas.