Being the pics are NOT of cases fired in the OP's rifle, how you determined "there's the answer" baffles me. Say by chance your guess is correct. Isn't he going to have to take it to a gunsmith anyway? All I'm saying is he should start with the gunsmith unless you can fix it remotely over the internet. I see you've changed your mind & are now saying the chamber can't be cleaned up even if the barrel is set back. So does that mean he needs a new barrel now? Which of your guesses is the OP to follow? Or should he wait till you come up with a third guess? Until you have that rifle in your hands, any analysis is mere speculation.There are no 'theories' when we see the picture that carp diem posted! There's the answer. The fire-formed case is WAY different than the unfired. With that much expansion in a low-pressure cartridge, the crimp can hold tight enough to jerk the case in half. 38-40 cases used to have a cannelure at the base of the bullet and it was common to break cases through that stress riser.
By measuring the fire-formed case, I think you'll find a chamber that won't 'clean up' even if the barrel is set back. Revolver cylinders are SOL.
Here is a picture of the cases. 1 with a rupture, 1 that didn’t and the factory ammunition. This is a rifle built in 1904.