Doing detective work to understand why the barrel had fouled.
Yes, there is one deep scratch in the middle of one groove. This shows the start of the scratch (after brushing).
The scratch follows the twist down the barrel. It's very consistent, almost like it was done with a machine. This is about mid-barrel. It peters out after about 16 or so inches. Well, the scratch certainly looks abnormal and it certainly makes me feel sad.
It wasn't there the last time I borescoped the barrel, so I'm guessing that somewhere along the way a piece of hard grit got embedded in a bullet and created the scratch when the bullet traveled down the barrel. That said, I can't think of any reason the scratch would hurt accuracy or cause fouling.
Most of the barrel cleaned up nicely with normal brushing, but there was this one spot at the leade that appeared to be a gob of lead.
So then I cleaned with Birchwood Casey lead remover cloth, followed by a JB clean, until the glob was gone. It appears that the problem spot has pitting. Bear in mind that the Lyman borescope magnifies the image, so in real life the pits are microscopic. When viewed through my Russian borescope, the pits are barely visible. Nonetheless the pitting appears to be the root cause of the glob of lead. I could not identify anything else wrong other than the pitting.
Recall that the previous chamber, the 308, had to be abandoned due mysterious damage at the leade. It wasn't clear if that damage was due to erosion or to something else, other than to say that one day the damage wasn't there and the next day it was there.
Then I set the barrel back and cut this 30x1.75" chamber, but now it, too has damage at the leade, and this time it does appear to be pitting caused by erosion. I get the impression that the metal in the bore has fatigued such that it is prone to pit easily.
-- Set it back once again, but if my theory is correct then the new leade may be pitted after only a few hundred rounds.
-- Limp along with the pitted throat. Perhaps if I cleaned it aggressively after every group, the accuracy would be decent.
-- Scrap the Shilen and install a new barrel.
I do have another barrel on stand by, a Hart 14" twist, but it was intended for the homemade actions that I hope to build this winter. I'll have to think about this for a while before I make a decision.