Secondary Explosion Effect? Or.....?

[woods]

Common occurrence in a case head separation

I've only had one but have seen other threads showing the same. IMO in this situation it is gas escaping out the separation at the pressure ring and moving forward out the bore or getting trapped between the case and chamber.

Others have postulated that a low load where the gas gets around the neck and around the case body before the neck seals might be the cause. I don't know but IMO it is obviously trapped gas

 

[Mikecr]

I only persist with the question because it presents an opportunity to learn the 'hows' of consensus.

Try this.. Maybe gas blows by shoulder, then case expands first sealing necks to trap the gas, then shoulders, then near webs. This, compressing the blowby gas tightly to the weakest point of a case body(near the shoulder). At that point the compressed blowby gas would be xx PSI above it's trapped pressure while in a smaller area. Then as bore/chamber pressure falls, and the case is still sealed, a point is reached where the trapped gas is higher in pressure than chamber, allowing indentation of a case.
With low enough pressure the case springs back from chamber walls as usual but exhibiting a strange dent.

How's that?
It's the only thing i can come up with

 

[Pdvdh]

Looking forward to a resolution on the sequence of events that causes these large collapses near the case shoulders.

The large deformations are uglier than warts. First time I saw one of them my eyeballs touched the lenses of my reading glasses!

 

[woods]

Do you think that chambers with a lot of freebore would be more prone to let escaping gas escape from behind the bullet and get around the neck more easily?

 

[Lefty7mmstw]

Do you think that chambers with a lot of freebore would be more prone to let escaping gas escape from behind the bullet and get around the neck more easily?

yep... the bullet is either closer to being out of the case as the pressure comes up or already exiting.
As far as the dent formation, I'm pretty sure it is there (from the trapped gas)during the firing sequence but gets bigger (to final dimension) as the pressure comes off.

Case seperations as in the above pic have a nasty habit of depositing a carbon ring in the chamber that should be scrubbed out prior to firing more rounds. The ring is unlikely to be even so it may cant the round in the chamber. If part of your case gets stuck in the chamber a pistol bore brush or (being extremely careful) a wood drill bit can often get it out. A bit of penetrating oil can help loosen it up.