I'm new to reloading and long range shooting in general. I recently had a 300 rum put together and loaded some base loads up for it. Using Hornady reloading manual and a base load for 180 bullet, I got one casing that had 2 major dents/creases starting at the neck and running half way down to the base of the case.
My gun smith initially got a few pressure signs with factory ammo on the rim of the case, but he was able to adjust the head space and correct the issue and had no more pressure signs after that.
i think this was shot #8 of the day and shot a couple more times after this casing and didnt see any more issues.
I have searched and searched and have yet to find an example and explaination on the web.
I've seen similar results when I attempted to seat long bullets (VLD's, etc) to magazine capacity in large/belted magnums. In other words, a load that normally showed no pressure signs when seated to lands or jumped slightly has cratered shell casings when the bullet base was seated below the neck junction. Obviously deeper seating creates more neck tension and greater friction, therefore greater pressures. But typically you will experience sticky bolt lift, cratered or loose primers, or other pressure signs as well. My advice would be to start with lower charge weights and work up in .5-1 grain increments while watching for pressure indications. Keep in mind that just because a load shoots well from my rifle, it may not be safe in yours due to faster twist, tighter headspacing, etc. And primer selection, seating depth, neck tension, etc. can cause large increases in pressure within loads.
I've experienced the same thing with my 7 RUM. Only one so far out of about 150 shots. From what I have been able to find out searching the internet it's caused by the .400" of freebore used in the Ultra Magnum chambers. When the bullet is seated out longer than normal the neck of the brass does not seal the chamber and it allow the gas to flow back between the chamber wall and the brass collapseing the brass. Or at least that what the consensus seems to be. You can find about as many explanations as there are responders. I'd really like to talk to a Remington engineer about it.
There is a similar thread on this page with a 30-378 cartridge. There are several possibilities, but I would be willing to bet that you made a very simple and common mistake when you begin reloading. It looks to me like the crater starts just above the shoulder junction. You may be getting too much lube on your cartridge, or your die vent may be clogged during resizing. This will create small dents in the slope of the shoulder, which will allow a seep in pressure from the chamber into action. Pressure theoretically should push the case out against the chamber and prevent this from happening when all is well. But if there is enough gap in headspace for one reason or another, the result will be blow by and a deformed case as you have. I have made this mistake myself starting out. Often, the shoulder will just appear slightly dimpled. Try cleaning your dies with a fast drying solvent like gun scrubber and blowing them dry. Then only apply enough lube to the case sides to keep them from sticking. If there is extra lube on the shoulder or neck after you pull it outdryer running a dry case through but don't force it or it will stick. If that's not it, try cleaning your chamber. Debris or moisture in the chamber could cause the same issue. Keep me updated.