I read in the "Glock Buyers Guide Personal Edition" that the Glock .40's are 1.18" wide (same as the 9mm's) and the G20/29 are 1.27" wide (same as the 45). I assume the wider frame accounts for the difference in the barrels. I guess I will just have to buy a 10.
I have a G20 (10mm) and bought a lone wolf conversion barrel to shoot. 40 S&W.
I have put a couple hundred rounds through it using the 10mm stock mags and have had no issues or FTF at all! I already had a SS guide rod in it. So not sure if that has benefited its performance.