Wellllllll.......
My favorite money man is Roland Manarin, Omaha, NE. [manarin.com and manarin on money.com]. Some call him "Buffet Jr." He was a small child in WW II Europe where/when armies were killing people, breaking things, not to mention militias... Got a real "hands on" look at economics from the bottom up.
The category you mention is called "hard assets." Gold? Coal? Copper? Lead? Guns? Cars? Good wine? You are betting on your judgment.
As suggested, if you get an FFL and "deal guns" buying wholesale and selling near retail... Most small businesses don't survive without about a 33% return. Now if you can deal from home [the basement bandit]... cuts costs... 10% might not be too unreasonable.
Mr. M likes a balance. Common stock [OWNERSHIP] for the return. Some gold to protect against inflation. And some, a few, treasuries to protect against deflation. (yeh, like that is likely until the big DUMP...)
Obviously you have to finance the operation. Overhead. Insurance. Lights. Heat. Security! Then you get to look at some guns. No end of stories about the "exceptions." Colt couldn't give away "P" peacemakers so they quit making them right after WW II. NOW everyone wanted one. Original Colts got "bid up" to sky high levels. Mr. Ruger came out with a decent product to meet the demand. THEN Colt decided to "get back in" and the original Colts "dropped like a rock." You want the gamble? Diversify!
The rest is courtesy of the government, called inflation. Uncle Sam has tried to take the bottom out of the business cycle. Only go "up." Been done many times thru history (doesn't work forever). Economics history. Dull but enlightening if you care about "your money." I think it was the Romans who punched holes into the center of a silver coin to get more silver to make more coins which were all valued at the value of the silver in the coin before the whole was punched... Government!
Point? Look at the Florida Land boom. (Marx bros "Going Coconuts" movie). Or the Tulip boom in Holland. Or the Chicago land boom. It all "busts" eventually, so don't forget to take plenty of profits off the table to live on when the "bust" comes. Many, reportedly, "lived well" during the depression off the dividends of gold mine stock like Homestake.
I thought along these lines and still have a can of 4227 (empty) with a sticker says $1.95 on it. Now if it was full. Current price? First Mauser I bought was $35.00. In .30/'06 no less. Value today? Before my time although I know men who remember it (darn stories)... Uncle Sam would drop an '03 on your porch REA (Railway Express Assoc?) for $18.00 and /'06 surplus ammo at $0.01 per round with the gun. Those deals are gone. Today? You have to search. Enjoy. Luck.