Re: Severely Dented Cases….Secondary Explosion Effect?
As others have already mentioned, it's caused by too light a load not fully sealing the chamber, and allowing gas to come back between case and chamber walls. Step up the powder charge a bit and this will completely disappear. Fairly common in very large capacity cases, especially with some of the slow burning powders. These propellants don't deal well with reduced charges, and this is merely one of the (nicer) ways they show their displeasure.