Just so it's clear, when an FFL runs a nics or pics background check on you there is no firearm information sent. Only your personal information is provided. The government doesn't know what you're buying, they just tell the FFL if you're prohibited or not. The FFL keeps the 4473 records with the firearm info, but those are only transferred to the ATF if the FFL shuts down or does something wrong. Apparently there is some massive warehouse in Virginia or something full of old 4473 forms that I hear about, but it is pretty unrealistic for the government to digitize the system and expect it to useful with long guns being passed around like hot cakes.
Also as azzy says, there are few secrets these days. Buy anything online? Talk about guns on a forum or in your email? Withdraw cash from an ATM at a gun show? Browse arfcom?
The thing we have going for us, is there are a lot of us that have evil black rifles and even more people planning on buying them. It would be a massive undertaking for the government to enforce rifle confiscation or registration, he'll they can't even enforce background check failures. That still doesn't mean it would be ok for them to pass any laws requiring registration or further regulations. Im just saying, our government seems like it can barely find its own ass in the dark with both hands sometimes, so I don't think the insurgents of pittsburgh offroad are real high on their priority list but hey, who knows... [adjusts tin foil hat] .... So I'm not too worried about buying a rifle from an FFL and adding to the long list of 4473s in multiple states you will find my name on. Good luck tracking that mess Uncle Sam.