I've tried the cold spray trick with an over sized bore brush - nope. Then I made two different cerrosafe chamber casts. Then knocked them out hoping they would pull the piece out - nope. I now have the bore plugged and the chamber filled with Bore Tech copper remover (ammonia free, and safe for barrel steel). Since brass is made of a copper/zinc alloy, I want to see if it will break down the copper component, and weaken the case neck's grip.
But honestly, I'm starting to wonder if it is a brass case neck. I cannot see a brass color to it, even on the edges. Maybe its just many years of age/tarnish. Or maybe its some type of steel sleeve, I'm just not sure. As a way to test this, I put a .45 ACP case in a small container and covered it with the copper remover. The liquid is starting to turn blue as expected from the copper dissolving. I'll gave the chamber another day or so to soak, then pour out the copper remover and see if it is turning blue. If it is, then I'll assume it is a brass case neck. If not, then it may be a piece of steel.??