Relying on the sanity and/or consistency of government policy would keep me up at night.
As for current NFA items holder, the constitution requires them to be compensated fair value if they are to be confiscated.
The risk is arguably lower than many single stocks, many of which are bought in retirement portfolio.
There's plenty of ways to not confiscate them but impair their value.
Further restrictions on transfer, restrictions on use, disadvantaged tax treatment, requirements for storage, security, insurance, bonding, etc.
NFA firearms have an artificially high value because of the exact set of legal restrictions that the government has put in place: loose enough to not crater legal demand, yet tight enough to restrict legal supply. This market is tied to within US borders.
The government can destroy the assumptions behind this market with a stroke of the pen.
Later you said lower risk than many individual stocks. Maybe, maybe not depending on how we define things. But I do think it's quite possible for the price to go down.
Where is that in the constitution?
Passing a law which you can challenge in court that says “machine guns are illegal now, turn them in so we can melt them down for scrap” is not public use.
The EO couldn't have forced an uncompensated surrender of a registered bump stock, were it one existed before the Hughes Amendment.
There’s plenty of flexibility in the case law for what counts as “public use”, but nearly all of it is about individual cases where the government takes a specific person’s specific property, or damages it in some way. There doesn’t appear to be much case law at all for the guardrails if the government declares an object to be illegal to possess writ large for safety purposes and requires owners to destroy or surrender those objects.
I’m not saying there’s no path where the courts would require compensation, but for the level of certainty you’re claiming, I’d expect there to be a more clear line you can draw to existing cases.
My initial claim in any case was that the constitution requires the compensation, not that there is 0% chance the government would violate the constitution.
None of the above has anything to do with the government violating the constitution.