Very few guns are a worth while investment. Very, very few.
Full auto machine guns, legal in 39 states, are the best guns to invest in. According to Forbes machine guns are the best investment possible since they have consistently out paced gold, silver, and all stocks. Part of the reason they increase in value is the 1982 Gun Control Act that closed the machine gun registry to new models and addition guns. The pool of guns in that registry are the only once legal to transfer from private citizen to another. A Mac-10 machine pistol cost $375 plus the $200 tax stamp in 1981. Today a 35+ year old used one sells for $4200 plus $200 tax stamp. In 1981 a stripped Colt M16 receiver was $67, today the same thing new in the box is over $18,000. The used M16 costing $400 in 1981 sells for over $9500 even with all the scratches and wear.
Most commonly sold guns - you will be long dead and your grandkids at retirement age before they appreciate in value enough to notice.
Some common pistol are good investments and do increase in value: Walther PPK,PPK/s etc will increase $40 a year..... so will top end Kimbers and Colt 1911's. I bought a new Colt Gold Cup 1911 for $500 in 1989, fired it for decades on several pistol teams - I put thousands of rounds through it - then sold it in 2010 for $900. Very few guns will do that.
Want to know what to invest in? Go visit a couple of guns shows - what ever you see on the tables will be guns and models you want to avoid!! LOL. You wont see any Walthers or Kimbers and damn few top Colts.
You also need to do research. Example: Smith and Wesson was making the Walther PPK/s for them in Maine. At the beginning of 2009 they ceased production to handle a recall. This made the PPK hard to find. Come Sept of 2009 Smith and Wesson was notified by Walther lawyers the contract required them to produce at least 60 of the PPK/s a year. Of course S&W freaked - with the production line closed for nine months - they had to do something. Solution was to assemble a bunch of master pistol smiths and hand make 60 pistols. 30 were sent to East Coast top distributors and 30 to west coast top distributors. There were called 'allocation pistols'. I happen to have one, in 2009 I wanted one for my wife to replace one she had sold a decade before. Last time I saw one of these it was languishing on gunbroker for $500 until someone noticed it was one of those allocation pistols - then it went over $1500. Finding one of those and paying for it like it was a regular PPK/s would be sweet. If you plan to invest in guns, but, do not want to have machine guns - you need to learn about things like the PPK's story. That means staying 'plugged in' with all the local dealers and clubs.