If the casehead separation is due to too many reloadings and the case seaparates because it is too thin above the web, then the only problem will be that gas and a bit of brass will blow back through the locking lugs and around the extractor. The gas and debris will then be deflected by the bolt carrier and will proceed down the magazine and back through the action. There will be little damage done.
Now, if the casehead separation is due to an over-pressure round, then all kinds of other things can happen. It all depends on how high the pressure is, and how quickly it builds.
ETA: For example;
In the picture above my post, my best guess here is that there was a bore obstruction just past the gas port. It appears that most of the gas went down the gas tube and blew the bolt carrier apart. The bolt is still stuck in the barrel extension which is cracked. The locking lugs are likely to be cracked as well, both on the bolt and the extension.
This might have been caused by an overpressure round too, but the peak of the pressure curve would have been later than say loading with pistol powder.
If the case were overcharged with too fast a powder then the bolt and/or the extension would be separated breaking the locking lugs. There is also the possibly of a bulging or bannana peeling the barrel at the chamber end.