I'm back to what I hypothesized at Post #43, that the cartridge head was swelling, enabled by the hammer not backing up the bolt. After all pressure is released, the swollen head is wedging the bolt against the cam. Not wishing to be thought a nag, but I still want to see rim thickness measurement before and after firing.
For that matter, what happens if you return one of the swollen cases to the chamber (I know it's a pain) and close the bolt. Do you meet resistance before the lever is all the way home?