I oppose guns are not for investment, but I will say it's not the best investment venture one should take. That said, if you are looking for a higher growth, guns are probably not the right choice. Buying a gun to toy around, provide you take care of it and sell it in a few years to making a little profit and that, I believe, is achievable.
Those mentioned in the earlier are good choices and you can also benefit from some of the higher grade European guns if the Euro keep on rising like the way it did in the past 7 years.
Any investment has some risk involved, usually the return is coherent with the risk and a bad investment would be if the risk is greater than the return so I would not consider buying a nice gun is high risk.