Sounds like Lone Wolf is the way too go. But looking through their website I am not seeing any of the .40 barrel for the 31. They have the 9mm though, but I really want a .40, since I have no other guns in that size.
Thanks for the rep. Remember what chezuki pointed out in the thread. You can use either a Lone Wolf barrel or a Standard Glock barrel. The advantage of a Long Wolf barrel is they are fully supported plus standard (standard rifling vs polygonal) and true drop in and you would be able to shoot lead bullets.