Those do not look like rifling marks to me. They look more like A. Loading from the mag box and dragging up the feed ramp and into the chamber or B. Single feeding doing similar as it seats into the throat. Unfortunately you cannot gently remove them from this rifle. If you could, they would probably be on the bottom side due to gravity.
"Tight chamber" to me sounds like, barely spec'd reamers to allow for max chambering jobs so as it gets closer to worn out, it still has life left in it. I am not a GS and do not know anything about anything so take it FWIW.
I would be curious to see if they actually head spaced this before it went out. I would be willing to say if ANY riflesmith had this come out of the shop, they would demand you send it back as it is their name on it. Christensen obviously isn't concerned with the explosive device you are placing next to you face.
I would get the name of the individual that gave you this advice and ensure you speak to the manager/owner if possible. His employee is fielding phone/emails regarding these explosive containing devices next to your face and stating it is normal. IT IS NOT NORMAL.
Christensen also states they do not warranty if you handload so if you cannot shoot the exact ammo that was designed around the 300 PRC or handload ammo for the 300 PRC, what does Christensen advise?
I too had a Christensen barrel, it was not chambered by them, but it did pressure up faster than I would have preferred.
I recently had another barrel company that chambered a prefit for me, refuse to acknowledge something wasn't right. It was slower than posted velocities, it pressured faster than posted velocities, was very inaccurate. Regardless of how big these companies think they are, they cannot continue to put out garbage and expect to stay around long. The internet crushes business these days.
Sorry rant over.
Glad you are sending it back. Too bad it was a gift, or I would advise having Christensen send you your money back and getting one from a company that stands behind their name.