The width of the lips on the mag along with the diameter and length of the cartridge determine the vertical position and angle of the cartridge. There is an optimum location and angle for the cartridge as it sits at the top of the mag awaiting its turn to be chambered.
If it sits too low, it may not get stripped out of the mag reliably and/or it may hit too low on the feed ramp and jam. An incorrect angle and an extreme high position can cause the same sort of Fail To Feed problems.
If it sits too high, it can interfere with the ejection of the spent brass. It may block the empty cartridge from being extracted reliably from the chamber and will usually cause it to either pull loose from the extractor and stove pipe or be re-chambered - Fail To Eject.
Since the .40 cal cartridge is larger in diameter than the 9mm, the 9 will sit higher in the top of the .40 mag - high enough to cause Fail To Eject problems but in most cases not too high to cause Fail To Feed problems.
Ideally, you want the CENTERLINE of the 9mm cartridge to be in exactly the same position as the centerline of the .40 for optimum reliability. The new XDm 9mm mag is designed this way, it may be both a few thousanths shorter and narrower at the lips.
The alternative, if you want to accomplish the same thing with a .40 mag, is to close up the lips just enough to lower the cartridge to the same optimum position. The trick is to squeeze or bend it consistently on both sides from front to back in order to preserve the angle of the cartridge as well.
It is probably not near as complicated as it sounds because there is quite a bit of tolerance there. However, I can always use another mag and since Scott was familiar with the procedure and was doing other work for me as well, it made sense to just buy one from him and get him to do it for me.
Last edited by dbfxdm; 10-30-2008 at 03:20 PM.