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.
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.