Is the fourth one from the left a 240 Gibbs? I have split some shoulders making mine. I always cringe a little when I pull the second trigger.
If you mean 4th from the left in my photo, I believe it is a Gibbs. Hard to tell the exact caliber.
I think what happened was - the cartridge was not loaded hot enough to generate the pressure needed to fire-form the case. Hot gas bled back between the neck and the chamber before the neck could completely obturate and seal the chamber. This gas buckeled and distorted the neck.
I’ve had similar things happen when fire-forming. Sometimes even the shoulder will buckle. Fire-forming should be done with a full power load when using a bullet or with sufficient fast burning powder when firing without a bullet.
The best way to fire-form a wildcat like a Gibbs is to first expand (or reduce) the neck so that it’s a good fit in the chamber. You can usually avoid split necks by annealing first.