I don't buy guns as an investment. To me every gun I have is a tool. I use them all, and I care for them as best I can. Having only sold one gun I made a little money, but it was an Enfield rifle I no longer wanted for deer hunting. I replaced it with a pre-64 Winchester 70 from a seller who did not know what he had. I got it for less than half its value.
Would I sell it for a profit? Not really, as it's a great firearm and does what is asked of it and more. It is far superior to something new I could have bought for the same money, workmanship being what it is today.
Any collectible can bring a lot of money, or lose money, or bring none. I used to buy and sell collections of GI Joes from the 60s and 70s. One suitcase of rare ones helped buy me a house in the mid 90s. Then the bottom fell out: old guys had good collections and needed no more or, frankly, were leaving this world. No one younger had an interest. Today I could not buy a good used car for what those toys would realize. I still deal in estates of toys and car parts, but there's a glut: heirs do not care for collectibles and whole truckloads are being dumped on a saturated market. Yet the belief that they'll "strike it rich" persists. Just go to any decent thrift store and you can score nice sets glassware, stainless, and dishes for a pittance. I'd never buy new stuff now. Just went to a junk shop and there were stacks of new Fiestaware, a few bucks per piece. I use that brand so it was a great find.
Guns are a little different as they are functional in a way that old toys are not and more valuable than cups and saucers. Guns are more like collectible cars. Yet tastes change. The car market is interesting because if you look at Prewar cars, except for the rare and bespoke, values have declined. There's a craze to do wild customs, but will buyers want them in a decade? Maybe not; consider the rise and fall of the custom van. So have many 1950s cars. The same will happen to many 1960s vehicles, as Boomers age out of the market. Yes, a 455 GTO Judge or E-Type Jag will retain value, but not a '70 Lemans with an automatic and a small block or a Triumph Spitfire in average shape. If you have a 70s Trans Am with that big bird on the hood, I'd sell it now unless you really want it for the duration: that car has peaked in value.
If you like old cars as I do, you can find great bargains on vehicles not hip with collectors or that have fallen out of favor--Model A and Model T Fords are great deals now, as are 50s and 60s four doors. Wagons, though, are HOT. Will they be in 10 years? You might apply that same logic today's guns, but given that all modern handguns and black rifles look alike to me, I could care less than figure out what might be worth money.
So instead of looking for future return, I look for a nicely functional piece that is not hip: think K-frames in .38, all-steel snubs, gently used lever guns from the 70s and 80s. They are well made and will last a lifetime. I still wish I'd bought that .45 colt Vaquero with a lovely case-hardened finish, for just under $400. It was in the case at my LGS with the ugly black plastic, which is what the kiddies want. Not too many folk into cowboy shooting come to that dealer. I'd never have sold it, since it would be an investment in fun.
I'll let my other investments support me in retirement, especially real estate. More time for shooting, then!