Guns can be an investment. If you don't believe me, then just look at all of the Form 4's out there that list the purpose for owning a machine gun as "investment". But, beyond that people have been investing in firearms for decades. There certainly are investment grade antique guns and shotguns.
Then there's the "banned gun" or soon-to-be banned gun speculative arena. There are folks that try to figure out what the next gun or gun related thing the government will "ban" and then buy them for safe queens. Heck, I've done that myself!
I'm simply at a loss to see any gun - new, antique, banned, soon-to-be-banned, machine gun, etc. as a good investment right now. Perhaps if someone has some inside information on a shotgun engraver that does work as good or better than Francesca Fracassi that is as of yet undiscovered and relatively inexpensive, I'd say that could be a good long term investment if you pick the right shotgun and the right engraving.
The thing is that there has been a significant run-up in prices across the board with regard to all guns. In fact, I'm considering liquidating some of my guns and using that money to buy into a Rhodium pool. I think Rhodium is greatly undervalued right now, but that too is very speculative.