Can you wrench the barrel off yourself and look yourself?
The thing you need to look for is pitting. Were there "bright" scrape marks on the case you extracted? If so, that chamber is definitely done. You can't have a pit that concentrates pressure or has a stress point. Chambers need to have a smooth surface to keep pressure tension consistent through the steel. Maybe get some long Q tips or cotton swaps and see if the fuzz gets snagged, indicating a sharp edge, which is super bad.
As for the cause, hard to say. Could have been some leftover Sweets that cooked for all that time in the safe, some other cleaner where they warn you not to leave it. Or a spider.
There's a possibility you might be able to set back and re-chamber, but I'd not count on it if the pitting is bad enough to snag cases.
