Don, you raise an excellent question. The truth is I don't really know. But, I do think that the following types of guns will continue to increase in value, even at today's prices: 1st Gen Colt SAA's; WWI and WWII 1911's; Colt Pythons; Most, if not all, machine guns; Older Hi-Powers; Hand-made shotguns; Superposed O/U's. Of course you have to buy them at the right price, but on these, the right price would just be a fair price - not some price an unknowledgeable person sell for.
I don't know why someone would buy "run-of-the-mill" guns for investment purposes - Yes, I'd agree, not a good investment bet.
I'd bet you could name some others that you believe have good upside. But, prices can go down too - this has happened to many of the ''80's "assault pistols" I purchased. No firearm is a sure bet; but then again no investment is - not even government bonds!
FWIW, keeping guns in your safe is much safer than keeping money in your safe (bank account, bonds and CD's included). It's a sure bet that your currency will be worth much less 20 years from now.