Here's my more simplified take on it:
Investing in guns is like investing in collector cars. Most of the ones that are
guaranteed to go up in value already cost a fortune. Even then, there's the chance that you may take a loss if you buy when the market is overheated.
Some milsurp C&R firearms have proven to be good short-term investments- e.g. cherry Swedish Mausers that cost $45 in the late 80s and are worth $350ish today, or unissued Russian SKS's that cost $60 in the mid 90s and are worth $400ish today. The problem with these is twofold: (a) you'd have to buy a bunch of them (and risk unwanted attention from the ATF) to make much money, and (b) it's not always easy to separate the good milsurps from the mediocre ones that really haven't ever gone up (e.g. Carcanos).
YMMV.
