It stopped about 3/8ths of an inch from going in and wouldn't go any further.
I ended up driving this thing out of the chamber with a cleaning rod.


Does anyone have the slightest idea what it might be?
The three things that look like O rings are, and it was a nice snug fit.
The step inside looks like you could drop some kind of rimmed cartridge in it, but it would be 2+" away from the bolt face.