Late to the diversion, but...
Anything collected as an "investment" only makes sense if it is truly rare - rare enough that its value is freely determined by market forces (specifically, auctions). "Book/catalogue value" is a comforting fiction, unless it is just the last price (or an average of recent prices) at auction. Also, rare enough that even when the community of collectors shrinks substantially, the supply is still well below the demand, which is probably even more concentrated among true enthusiasts than before.
Cars, guns, liquor, stamps, coins, etc.
For example, postage stamps. For decades some collectors were in the habit of buying sheets of stamps (as in, several sheets) as they were issued, thinking their mint condition stamps would be worth even more some day. But most collectors want only one, or a few, of an item, and the number of collectors (of all kinds of things, really) has fallen. So pretty much any mint condition postage stamp issued after roughly 1950 (in Canada), except for true rarities and a handful that had high face value for their time (eg. 1$ stamp when ordinary rate was 4 or 5 cents), is worth only its printed value.
Thus for firearms: worldwide quantity of a particular firearm << number of hard-core gun-collecting enthusiasts, and it might be considered investment grade.