I have no experience with DRs......

MTBW....I have known 12 bore shotguns with buck shot or slug to do this. These examples are from India where ammo was difficult to get hold of & the reloading techniques were not standard. In the 3 or 4 cases I am aware of such opening of the breach with a shot, the plastic case was crimped very hard but the shots or slug was a bit loose in the case & the front wad was pushed down. In other words - when the trigger was pulled and the primer was ignited, the shot moved & then came up suddenly against the hard crimp before it moved again. This resulted in a pressure spike & the breach opened up. This was also known to happen with Shakthiman ammo (made by a private company with political clout).

Regarding the Chapuis rifle in this thread - I am still not clear if it was reloaded ammo & if the ammo was crimped.

The fact that there was case separation confirms that pressures were too high for what ever reason - high powder charge, tight throat, hard crimp, etc.....My guess is that the thumb lever might not have been moved at all & the action opened up as a result of the excess pressure. I would send the rifle back to the maker & have it checked with the same lot of ammo.