Buy the 40 1st, Storm Lake makes the 9 barrel. It's awsome, I bought the combo for my FS. I just put a 40c on layaway and plan to do the same. All you will need is a barrel and a magazine. Everything else stays the same. The way I understand it is you can can go from the 40 to the 9 but not the 9 to the 40..

Happy New Year

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Jack C