From the PX4 Compact thread, but posted here because it pertains to the full size...
I picked up one of the $60 full size 9mm barrels from eBay & a 20rd 9mm magazine to try in my Type
C .40. It feeds & functions fine. The barrel is tight when in battery, but does not line up centered in the slide vertically. I was only testing to see if it worked without shooting at a target to see how far off it is.
I'm guessing that WAL machines a spacer to go over the 9mm barrel when used in a .40 slide. Freeze the barrel, heat the spacer, and quickly pressing them together should be more than sufficient to hold.