I too have found gun collecting to be very habit forming, at one time I also collected cartridges to try to curb the appetite. Good books on guns has helped the fever, I have a book on Mauser rifles, I can look at all the different guns in it and not have the expense or storage problems with the real thing. I do have a Yugo M-48 in the safe for the true 3 dimensional experience as needed.
A good example of the investment aspect is my Russian SKS in laminated stock. It was my main emergency rifle for 6 years until I decided my AK-47 is better suited for that need. When I bought the SKS new in the box, I paid the extra for the laminated stock, even though the solid birch wood stock version felt better to me, but my gun buyer's instinct felt that the nice visual equities of the laminate would be better come resale time.
Most of guns I have sold off fetched prices in used condition, at, or higher than what I paid new in the box 5-15 years before, and I managed to enjoy shooting them for entertainment too! Can't beat that for recreational returns. :-)
Now I just need to find out what a low mileage Russian SKS with laminate stock in excellent condition is worth. Usually I cruise the guns shows, and will try to sell it next Winter, but unfortunately some guns become rare and gun show visits have proven fruitless in seeing certain types, example, remember all those Egyption Rashids and Hakims from the late 1980's. I have not seen a Rashid in many years, I have no idea what one would be worth.