I haven't experienced your exact problem, but I had something similar happen with a brand new Shilen Savage prefit. I had trouble setting up my seating depth data using the Hornady tool. The test case didn't want to chamber easily. Likewise, chambering ammo was difficult and extracted cases showed scuff marks on one side about halfway down the case, similar to the marks on your bullet. My ultra cheap borescope wasn't good enough to see what was wrong but it seemed as though there was a rough spot in the chamber, most likely an artifact from the chambering process.
I fabricated a home made device to hone and then polish the chamber; problem solved. I find that a bit disappointing because I paid for a "match select" barrel and it seems as though a rough spot in the chamber should not get by their quality control process. The good news is that once I honed and polished the chamber, the barrel now shoots very well. I regret not using a good borescope so that I could know for sure what the problem actually was.
Perhaps you have a similar defect in the throat area of your barrel. I'm not a gunsmith, I just pretend to be one in my reloading room so I can't offer advice on how to correct a potential defect in the throat area. You might want to consult with a real gunsmith and/or take a look with a real borescope to see if you can detect a flaw of some kind.
You say this is your back up barrel. Is it new or have you used it before? Did it work OK before? If so, it seems doubtful that it's a fault of Shilen. If it's new, perhaps they can help. Have you called Shilen and/or sent them your photo? By the way, nice photo.
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