I have both the 357sig and 9mm conversion barrels for my G22. I really enjoy the 357sig barrel. Shoots great, ejects great, no malfunctions. The 9mm works fine but kicks the brass right back into my forehead. 
Glock 9mm Conversion Experiment
I've got several .40 cal Glocks that I picked up cheap at Southern Connection. All except for one have been converted to 9mm with Lone Wolf .40-9mm conversion barrels. Drop in a barrel and add a 9mm magazine and it's ready to go. I read online that in a pinch you could just use the .40 magazines. I decided to try that with the latest one I picked up, a Gen 4 G23.
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I loaded up a couple of mags to see how the 9mm fit in the .40 mag. The feed lip appear to barely hold the 9mm. The round sit a lot higher. I was concerned that the spring pressure from a full mag would force them out but it never happened. I ran over 50 rounds through it had never had a single issue. I would have guessed that it would have jammed with them sitting that high but it never happened.
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I loaded up a couple of mags to see how the 9mm fit in the .40 mag. The feed lip appear to barely hold the 9mm. The round sit a lot higher. I was concerned that the spring pressure from a full mag would force them out but it never happened. I ran over 50 rounds through it had never had a single issue. I would have guessed that it would have jammed with them sitting that high but it never happened.
