I've thought about this a lot.
They are a unique investment. Not just 'monetary' but peace of mind , sense of freedom, and buying something today that may not exist tomorrow.
1) Peace of mind. I buy guns, frankly, because I can. Probably a handful of nations where you can buy 95% of desirable firearms for less than 1 weeks' pay for the average person. AR, AK, revolver, semi-auto handgun, pump shotgun, you name it, and it's probably available and affordable. I get a huge satisfaction with the purchase alone, a sense of freedom. Priceless. That's a whole lot more rewarding than buying golf clubs, a new water heater, new car tires, or a bank CD.
2) Better than most things you can buy or hobbies in which to invest. I will preface this by saying I once worked as a stock broker, had my series 7 and other financial and insurance licenses and was very experienced in the industry. My clientele were >$1 million in assets. I've seen very wealthy people lose EVERYTHING (not on my advice, by the way). One client turned an empire into a sports car (went from $33,000,000 to $500,000 in a few months in 2001-2002). Poor diversification. So, you could put your money in the stock market, and risk losing it all. I view that markets as a shell game, where the rich get rich, and the rest of us get the table scraps. About every few years, or at least every decade, the market climbs, and then collapses. People lose entire life savings. No thank you. Your bank gives you about 1%. Paying off loans is smart, but super boring (necessary evil of life I suppose), earning you the interest rate of paying the loan. Best to remain as debt free as possible.
Every hobby has expenses. And unless you buy used at garage sales, you're going to drop a few hundred to a few thousand on any hobbies equipment (skis, bowling, archery, computers, travel, biking, motorcycles, cameras, watch collecting, etc.). Most other hobbies equipment becomes worthless or devalues significantly after purchasing. Average guns at least generally hold their value.
Wild swings in guns/magazines/ammo mean that you can time it and buy on the dip quite easily if you pay attention. Right now, guns prices are low and it's time to buy. Ammo is high, so time to sell.
3) Buying a ban-able item. Today you can buy something that anti-gunner want to ban. How many of us would love to go back in time to buy something that is not available today due to laws/regulations etc? Enough said.
4) Survival items. Great sense of security having a safe full of guns and ammo! If SHTF, their value will be more than money.
You could speculate on magazines and ammo for instance. Right and even during Sandy Hook, the first few hours, I loaded upon AK mags at $5 each, and several spam cans of AK and Mosin ammo for real cheap. I seriously considered, but didn't dropping $10,000 into magazines and ammo. Over the next couple months, with a little work, I could have doubled my money or perhaps tripled it. I simply didn't have time to invest... but someone that did have some time and cash, could EASILY double their investment ...
At the end of the day, let's assume that your speculation on an 'investment' (after all, all investments are speculations) goes worthless. At least with guns you have a useful, tangible item.
1) Home - a place to live.
2) Property - something to use/farm, etc.
3) Money - well, it's useless paper.
4) Gold/silver/copper - holds no practical use. If you bought at the highs, you would have lost your behind on these in the last few years.
5) Sporting goods - eh,
6) Cars, motorcycles, etc. These nearly always go to zero. Useful, very.
I suppose as a traditional investment, you'd have to be willing to sell your guns and ammo, at market highs. Generally that's when you may need them. I am not inclined to do that, so they may be poor investments for me... there's the emotional or psychological point.