Correct me if I'm wrong, but I'm assuming this is not a new gun, which means the manufacturer's warrantee may not apply to you. You should check with Kahr to find out for sure and, while you're talking to them, ask them what options you have with them. Perhaps they'll do an inspection on the barrel and give you a recommendation, whether that be to leave it be, polish it some, or replace the barrel.
As for this being a "lemon", I wouldn't go quite that far. A lemon is something that's operationally broken or requires excessive maintenance and repairs. This defect/flaw may not affect operation or accuracy noticeably at all and may not require any repairs.
It does not appear to be raised, from your descriptions and pictures, and appears to be centered on a rifling as opposed to the edge of a rifling. Which means bullets will pass right over it without notice. If anything, it may require some polishing with a lapping compound to remove any slight raised edges along the gouge.
I'd personally test fire it and see how it shoots. If you have not other issues with the gun and it shoots well, then consider it an easily identifiable, unobtrusive character mark that can be used to identify the pistol if it's ever stolen and had the serial numbers ground off.
(Don't laugh...that actually happened to someone I knew in the Navy and he recovered his stolen pistol from the police by positively identifying such character marks even though the serial number had been removed.)
