If you really know guns you can get some great returns, IF YOU PLAN ON SELLING THEM AT A LATER DATE. If you hold them until you die then your wife or significant other will probably take a big loss at your estate sale ;-)
I've been in the firearm retailing/ffl buying/selling/trading business off and on but mostly on since 1991and I've read most every gun digest, shooters bible, blue book, magazines etc. since I was 12 in the early 80's (yes I'm a gun freak) and I have an extensive gun library. I attend SHOT show nearly every year and have had a booth there three times as a vendor. I've never taken a loss unless I needed money right now and had to sell immediately. Even then I only sold for a little less than I paid for.
The best thing that happened for me is that when I was a teenager I worked for and was mentored by an older gentleman who was the best gun/horse trader I've ever known. He taught me the right way to buy, sell and trade.
I've had more than a handful of guns that I've made a couple thousand dollars each and many that I made $500 or more. Like anything else you need to know what and how to buy. Run of the mill off the store rack guns will usually break even maybe less but they always hold value and are a great liquid investment. Just like everything else the internet has really changed the business, some for the good and some for the bad.
With all this being said, it can take a lifetime to learn everything even when you're specializing in things like S&W, WW2 and other types of guns. Are you looking at an original or something that has been altered, restored or faked when it comes to older or collectible firearms. It's easy to get burned and there are scammers.
It's the same with cars, antiques and other things, if you know what you're doing then you can make money. One thing about investing in guns is that they are a much more fun investment than the stock market.