That's actually been my habit to borescope every cleaning, but admittedly I'd been slacking on it with this barrel. That said, since my troubles began I'd been scoping every time I cleaned it.
Cleaning the snot out of the barrel was my first step after the TX state match. I hit it with CLR, bronze brush, then Eleminator, bronze brush (repeat multiple times over). After I had it as clean as I could get it with those products, I hit it with two patches of IOSSO (focusing on the throat, then taking longer strokes). The thing was looking brand new aside from the mild fire-cracking you'd expect.
If anything, I was worried I'd cleaned the barrel too much.
That was before this past Saturday's testing, which included re-running an OCW of N150 (the powder it had been running), and Varget to see if I could get them to shoot.
I just finished doing a mild cleaning and don't really see any areas of concern. There's no carbon ring, and no hard-carbon in the first ~6-8 inches of the barrel. The steel has a darker tint in that area of the barrel, but I am 100% confident if I ran some JB or IOSSO in there it'd shine up like the muzzle end. I pulled slightly more copper out than I ever have before, but I may have over IOSSOed the damn thing Friday night as part of the 'troubleshooting'.
I'm probably done messing with it. I still have 2,200 of the same lot 200.20X bullets for this barrel, and I really don't want to waste more of them trying to diagnose the barrel when I could just re-allocate them towards the Bart or Brux.
Sometimes the best way to troubleshoot a barrel is to stick it in the closet for a while, and pull it out a few years later.