Investments make money only two ways: they earn an income; and/or they are sold for more (accounting for inflation etc) than they were bought.
Guns, ordinarily, can't earn an income. That leaves capital growth. Sure, some have made good money buying cheaply and selling later at a profit, but what dividends did they forego by having their money tied up in guns? How much was eaten by insurance, storage, security, maintenance etc?
The only way I see guns being a worthwhile investment is to find sleepers and valuables that the vendors don't realise are valuable and buy them for a bargain. But that's arbitrage. It doesn't often happen, and the time and learning and legwork that goes into making those moments happen has a cost all of its own.
For those reasons, it's my opinion that investing in guns is a mug's game.
My guns are not bought with a view to what they'll be worth down the track. They're bought because I enjoy and appreciate them.
That