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.
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.
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.