I've seen this barrel and know both the shooter and OP. Both experienced shooters. It was a round ball. The dark spot a few inches back from the muzzle looked like old rust to me. As mentioned a few posts ago, possibly an old short start minor crack or possibly just bad metal. There is also a small lengthwise crack from the rear of the barrel forward for maybe a half inch or so. Bad metal or too tight of a breech plug?