Mortaring the rifle yielded nothing but a ruined charging handle from me banging it on a table when mortaring didn’t work.
Soaking in Kroil for two days and tapping the back of the BCG with a punch freed up the stuck case.
I wasn’t there when the smith got it apart, but the rim was still partially on the case, he said. He removed that and used a threaded tool (like the one for cases stick in a die?) to get the case out of the barrel.
