Looks like the case mouth didn’t seal on the chamber and pressure blew back around the case in the chamber and collapsed the shoulder. Since it’s a belted magnum case I’d guess that there was a healthy gap between the chamber and the case shoulder.
A collapsed case is a common sign of a low pressure load with a slow burning powder.
First time I seen a case like that was about 1965 when the 264 WM came on the scene, Some shooters using Hodgdons Army Surplus H 4831 working up loads with light bullets had this problem.
Check your load against published loading data, For low charge weight of powder.
If a new wildcat cartridge, Up the powder weight slow to get proper pressure.
Be Careful and Good Luck in search for a cure.
This has happened to me when I chambered the then new 300WSM case in 25 cal, the 25 Pronghorn it was called.
The simple fact is that the powder just doesn’t burn normally, it starts to burn, fizzles out, doesn’t seal the chamber then the charge goes off with a bang.
We think what’s happening is that the bullet leaves the case, then the gases are able to get around the bullet and pressurise between case and chamber.
I see your case looks exactly like mine did, sooted all the way to the case head and a large dent below the shoulder. The only powder we didn’t have this happen with were double base powders like RE22 and RE25, most single base powders did this with start and middle loads for us, H4831 being the worst of the lot...I know longer buy this powder as over the pressure trace it was a poor performer for me, never got the velocity it should have at the pressures given.
I built 2 identical rifles in 25 Pronghorn, neither of them lasted 12 months before the barrels were pulled.
Not always the case. We had this happen within 2g of the predicted max charge, especially with bullets of 100g or less, but it even happened with 120g Partitions with H1000.
When the 7RUM came out, and Remington had those 3 levels of ammo you could buy, we tried to duplicate the lower recoiling loads using canister powder, first was Benchmark...bad idea. Locked the bolt solid and broke the extractor, then H4885 and we had the same dented cases and abandoned the idea. We pulled Bullets on those factory Rem rounds and it was lucky to have a 2/3 fill of whatever ball powder they were using. If the 20 we fired in a production gun, 15 or 16 were sooted past the shoulder, velocity was as advertised.
Anyway, it’s not always a light load that does this, it has to have certain parameters in the load/bullet/powder recipe for it to happen.
Mine definitely weren’t short, within .010” of chamber end, like all of my brass. We even had new brass exhibit dents with certain powders.
Necking down 300WSM to 25WSM posed some problems for us too, necks were excessively thick and required turning and trimming. That rifle is now a switch barrel with 270Bee and 338WM barrels, still have that 25 Pronghorn barrel somewhere gathering dust.
If it is not low pressure loads then we would need a lot more detail to help you solve your issue.
Cartridge? Brass manufacture? number of firings? Powder? What gun was it fired in? Barrel? All the details you can? Without question you are getting gas blow by causing the case dent and the brass is not sealing the chamber properly. It still looks like a load of slow burning powder that is not pressuring up in time to seal the chamber. That could be from other than obvious reasons but we would need lots more details.
That line right above the belt in the soot of the case shows that your were a c-hair away from a case head separation.
If the bulge above the belt was large enough depending on your chamber, the case could have been slightly crooked in the chamber. A lot of people don’t like belted magnums because it’s a little harder to resize the area above the belt.
7 rem mag. Win brass, 3rd firing, retumbo, Stiller action, 26" benchmark 1/9 twist, 215 primer, full length size 5 thousandths, trimmed to spec length on gracey, this powder load shoots a 5 es, I was doing a seating depth test and this round was seated the longest at 5 thousandths off the lands. This was the first round fired of 15 in the test. No other rounds showed any signs of deformation one other had powder burns down the side. I'm starting to think this may have been an anomaly.
None of these shot very good. Best group was .7 most groups had 2 touching and a flyer. Time to swap primers?
With the great es I was hoping this was going to be easy to find a load.