Well, I don't have many guns, and
none are illegal, so I don't care much one way or another about having to register. It doesn't seem much different from registering a vehicle.
If there was a law requiring me to register my motor vehicles, and I had one that I just have as a spare/used rarely/junker (or vehicle I use only on my property,
if I had a lot of land) and didn't want to register anyway, I just wouldn't. If you could legally posses the firearm regardless of registration, it
wouldn't make much of a difference in penalty if you were caught. It is
simply a failure to register, and any penalty
would have to be proportional to the offense and prior convictions.
"The forfeiture of respondent’s entire $357,144 would be grossly disproportional to the gravity of his offense. His crime was solely a reporting offense. It was permissible to transport the currency out of the country so long as he reported it. And because §982(a)(1) orders currency forfeited for a “willful” reporting violation, the essence of the crime is a willful failure to report. Furthermore, the District Court found his violation to be unrelated to any other illegal activities. Whatever his other vices, respondent does not fit into the class of persons for whom the statute was principally designed: money launderers, drug traffickers, and tax evaders. And the maximum penalties that could have been imposed under the Sentencing Guidelines, a 6-month sentence and a $5,000 fine, confirm a minimal level of culpability and are dwarfed by the $357,144 forfeiture sought by the Government. The harm that respondent caused was also minimal. The failure to report affected only the Government, and in a relatively minor way. There was no fraud on the Government and no loss to the public fisc. Had his crime gone undetected, the Government would have been deprived only of the information that $357,144 had left the country. Thus, there is no articulable correlation between the $357,144 and any Government injury."
- United States v. Bajakajian (96-1487), Pp. 14-17.
That's on top of the fact that his Executive Order doesn't actually have any teeth whatsoever, because it's not a federal law passed by congress and registration of firearms has always been a power delegated to the states, with the exception of NFA weapons.