Oh, definitely don't try to drop in a stock Glock 9mm barrel into a .40 gun! Buy a CONVERSION barrel designed for the .40 slide, but with a feed ramp and chamber for 9mm! A bonus feature on most of these conversion barrels is that you can now shoot lead bullets (the unique rifling on stock glock barrels can lead up and cause problems wwhen shooting lead bullets)! Might invest in the proper extractor, though I hear the .40 extractor works well with 9mm conversions.