Upgrading - What to do with old gun + billions of accessories?

DreQo said:

... I remember a conversation with an M&P owner, in which he mentioned that some S&W M&P calibers shared the same frame and slide, with only a barrel and magazine being the difference. Depending on how interchangeable they are, I could theoretically get 2 or 3 calibers for the price of about 1.5 guns.

Can anyone give me the run down on this? It's something like if you buy a .40, you can get a .357 sig barrel to drop in, and maybe a 9mm, too...all though you may need a different slide for the 9mm as well.

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