It’s true that “unpapered” guns are preferred to those on a 4473 form, but recall that this only `registers’ the gun at the point of initial retail sale.
There’s not a federal law I know of that requires you to notify the authorities when you have sold, transferred, or otherwise disposed of the firearm listed on that 4473.
“In order to have a registration law, there must also be a deregistration law”
If they come to your house, you can say,
“I sold it for cash”
“I threw it into the lake/river/ocean just to keep you from getting your hands on it”
If they search your house and come up empty-handed, what is your crime (unless martial law is in effect)?
Some states now require all firearm sales to go through an FFL, just FYI.
If you live in such a state, move immediately.
Another clever one that someone else thought of:
After getting the word and hiding your guns, cast several thousand pound concrete blivets in your front or back yard. When they come by, point them to the blivets and inform them that the weapons they seek are probably in them. Watch the fun - most ATF search teams will not be equipped with cranes or 5 ton trucks.
Bingo
“I threw it into the lake/river/ocean just to keep you from getting your hands on it”
If they search your house and come up empty-handed, what is your crime (unless martial law is in effect)?
I am quite certain that it is a crime to knowingly lie to a government official. Thus, as is almost always the case, saying nothing is your best defense.
If you sold a firearm, best to have a receipt of some kind - and if you claim you sold the only gun you had in .308, for example, and "they" can come up with proof that you continued to order .308 ammo, reloading gear or components, mags, etc. after the date of the supposed sale...why, then you'd be in trouble. Best to have solid proof and not lie.
Saying you lost something is more credible if you have a police report or an insurance claim to back it up - but don't commit fraud of any kind.
The best advice I've ever heard is to acquire what you need off paper.