If GLock decides to come out with the single stack 9mm, it will be heavier, thicker, and bigger. It will not be the same size like the g42. The grip frame on the G42 is not even long enough to handle the 9mm round. The barrel is too thin to handle the pressure of the 9mm round. They will have to design a new pistol.
The Glock 42 is in a class by itself and has little in the way of competition. There are other options for micro pocket pistols but G42 is one of the few 380 mid size pistols in the market with great trigger and easy to shoot all day. If they would have brought out the 9mm first the .380 wouldn't have sold nearly as much.



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