I think that demographics are really against gun collecting as an investment. There are way too many guns in the hands of guys 70+ years old--their heirs are going to want to liquidate a significant portion of those guns over the next 20 years. I just don't see the next generation of buyers in place to keep those values at their current levels, let alone increase. The next generation does not have the disposable income or the fascination in them.
The 30 and 40 year old guys that I know who are into guns just don't have the same passion for the old walnut and steel. I just don't see that many guys paying many thousands of dollars for a gun they have no particular attachment to, in a chambering they never heard of, and know that they can never shoot it--if they want to retain the value.
