You're forgetting the cylinder gap. I've seen similar cross sectioned revolver barrels.
It seems with revolvers it's possible to stack multiple rounds in barrels after squibs because the pressure is vented through the gap, which out otherwise blow out catastrophically after the first shot in a design without that quirk.
I also remember a story about a hi-point carbine that got returned to the factory (and subsequently refurbished) that had then barrel stacked full of rounds. It seems that blowbacks are sometimes also able to survive multiple stuck rounds by nature of not being a locked breech.