I have 2 XD40 service models and one EFK 9mm conversion barrel. One of my XD's has a CC trigger job, DGR, and a 22# spring in it, roughly 2000 .40 rounds through it. Using the 9mm barrel on that XD works flawlessly. So well that I use it regularly in IDPA without any problems whatsoever. I have over 1000 rounds of 9mm through it already. The other XD only has about 750 rounds total through it, and does not like it quite as much. It fits much tighter in the slide, around the chamber. It could be just a break in type issue, the gun might just need to be losened up a little. Also, the 9mm barrel is more accurate than the .40 barrel.
With the 9mm and the 22# spring, this thing shoots like a kitten. Has the recoil of a .22 now. I absolutely love it. Offhand shots are far more accurate for me now, especially with quick follow ups.
Hell, I trust it so much that right now I have it in my nightstand loaded as a 9mm with hollow points cause I shoot it better than the .40, and I have 17 rounds instead of 12. I trust my life to it. But only after it proved it's self.
Those who will sacrifice liberty for security, deserve neither!
If you shoot a mime, do you have to use a silencer???