Originally Posted by 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?
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.