A similar subject brought me here to begin with. I was (still am, actually) considering sleeving a spare Kahr barrel to .380 for my K9. I would have two or three dedicated mags with a small spacer at the rear to insure the cartridges fed correctly.
What's the point? I have a LOT of .380 ammo, and it seemed as though it would be much more pleasant to shoot from a locked breech gun. No more, no less. The K9 stays 9mm capable, and with a quick barrel/mag/recoil spring change, it is a .380 as well.