Having burned up many, many jugs of Varget in 308s from 155s to 200s over the last 20 years or so, it's really not an issue.
Having and using a borescope isn't a terrible idea, but of all the things that might be going on with this guy's rifle, a carbon ring is one of the very last rocks I'd be turning over.
Fair enough, you might be right.
But whether first or the last rock to turn over, it's a rock that needs to be turned over.
That said, after thinking about it, and based on what's been said, it seems like f'ed up headspace (as in too much bump) is the more likely cause. With what the OP has shared, as far as him being green when it comes to sizing dies and bump, I'm leaning more that way. Especially since I remembered him having a "dud" (FTF) in one of the vids he posted...
Like a lot of guys, I've shot a lot of f'ing rounds, and "duds", as in bad primers that don't go off, are rare. But bump the shoulders back too much (even by just 1-2 thou too much) and FTFs become frustratingly and annoyingly common (been there, sucks lol).
I think a lot of guys new to reloading don't understand that if one's bump isn't going to be consistent and repeatable, and possibly one might be bumping too much (which is what it sounds like here), it's actually probably better to not even bother bumping the shoulders back at all until you can nail it.
FWIW/FYI to those that don't know, when setting bump, it's based on the longest measured cases, and then just running the shorter cases through the same die after it's been set and locked in with a lock-ring. Do it right the first time, once, and then you don't have to touch it again for the life of the barrel (in other words, it's not something you have to measure for and adjust every cycle).
Based on what's been shared, now he's probably best off finding the longest-shouldered ones, and setting his die to not bump them back at all for a cycle or two (to let all the cases grow to a uniform length). Then, after a cycle or two, he can find the longest-shouldered ones and bump them all back his desired -.001" to -.003" like a seasoned pro (and probably fix a bunch of this wonkyness he's been experiencing).