He didn't state clearly that he knew the case was his; just that it was in the brass he was sorting. But the diagnosis is correct: unburned powder in the chamber. That's a good sign the powder is too slow for the bullet weight and not burning consistently, as it likely spilled out of a case while it was extracted. Using powders that are too slow can lead to ringing the barrel, so if it were my case I'd be looking at something faster. Maybe Reloader 10X or Benchmark or H322. I've used Varget with 77-grain SMK's without issue, but never tried it with anything lighter in 223.