Kal,
This is more than a matter of merely having an old unregistered gun (not even a misdemeanor in most locales), it's a matter of possessing an unregistered NFA firearm, which is a federal felony.
The advice you gave was to violate federal law, and regardless of whether that would be your choice or not, it remains bad advice.
Can you get away with breaking the law? Sure, in one form or another millions of people get away with breaking thousands of laws every day. Is this one worth breaking? Not to most of us. The risk is simply too great.
Think you can just hide it & feel safe because the cops have no reason to search your house? Think again. As I mentioned, circumstances you'd never even imagine can legally bring cops into your home. Been there & done it.
You can certainly make your "Keep It" post, but that's why you're not getting a lot of support on the idea.
And, should such a gun come to the attention of the ATF, they'll go to town on it. They have about zero sense of humor on such matters. There might be a plea bargain worked out to avoid jail time, but it'd certainly rack up a tidy legal bill, and would most likely result in the loss of future gun possession rights.
Denis