Smallbore has posted some time ago his experience of firing an unlocked Jones rotary underlever double with the lever open. In other words the gun was held closed by mere hand pressure. He did not notice anything untoward.
The forces acting on a double during and immediately after firing are a bit of a mystery. Even Autocad stress simulation does not adequately show the force development, it leaves out the friction component between shell case and chamber for instance, at least in the versions I saw.
How that piece came loose in the OP Smith seems, to me anyway, to be a result of pressure or corrosion than strain on the action from normal firing cycles.
