As long as the barrel and slide are in full battery (which I suspect they are), I believe that you are correct in that the slide sits forward on the frame because of how the barrel lug is cut. This doesn't necessarily mean the barrel is bad. It's quite possible that the pistol will function correctly as is. As a quick check, operate the slide briskly by hand and observe that the recoil spring guide rod stays in place. Then drop a pencil/pen/dowel with a hard flat end into the barrel, tilt the pistol so the pencil/pin/ dowel rests against the breech face/over the striker hole and pull the trigger; if the slide is positioned correctly the striker block is operated so the pencil/pin/dowel should be pushed from the barrel by the striker hitting it. If this does happen, I'd be inclined to test fire the pistol. If it works, then the issue is cosmetic.
The above isn't meant to discount the chance that the frame could be bad (holes not properly placed as mentioned earlier, excess polymer at the rear, etc), so you might want to wait for Kahr's response before going forward.
FWIW, the slide of my CW45 sat forward of it's old frame a slight amount and quite a bit more forward on the replacement frame.
As I specifically asked that the old slide and barrel be put on the new frame, I'm not surprised that there was some mismatch. I wish the fit would have been better, but the pistol functions perfectly and is spot on accurate.
You can just make out the issue in this photo:
To make the mismatch more palatable, I slightly beveled the rear of the frame:
Regards,
Greg