It will fit in physically, but should not be used because the breech fit won't be correct, and will give you problems.
Be aware you may have to change the extractor/ejector also if reliability is not adequate with the conversion barrel alone.
If you want 9mm out of your G23, another option is to go to .357 Sig, which uses a 9mm bullet at higher velocity.
The .357 Sig barrel will drop in, and the breech matches the .40 so you won't have any issues with ejector/extractor mismatch.
(The .40 is the parent case for the .357 Sig, so the case head dimensions are identical, which alleviates the potential reliability concerns of going to 9mm in a .40 frame.)
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Last edited by RottnJP; 07-04-2012 at 05:38..