The Glock factory .40 & 9mm bbls do not interchange between the slides. .40 slides have been beefed up on the inside, making them thicker than thye corresponding 9mm slide. Lone Wolf makes bbls that convert .40 to 9mm. But the width of the breech face can affect reliability of the extractor.
I have a Glock 19 slide/bbl that drops onto the G23 frame with no mods. The ejector is different on the .40's and 9mm's, but I have never had a malfunction using one with the other.
My carry G17 came with a .40 ejector (used) and I never bothered to change it. It has beaucoup thousands of rds through it with no malfunctions.
Last edited by M14sRock; 04-11-2010 at 05:36 AM.