When I test fire the first round ever to be shot in a new barrel and chamber I Do it remotely and start with a moderate load and carefully inspect the fired case looking for any marks or anomalies compared to an unfired case. if everything is good, and the chamber is cut properly the case will look as good as new. I also take a loaded round and check to see if the bullet can be inserted into the case mouth of the first fired case. This will tell you if the trim length is insufficient causing the chamber to crimp the case mouth.
Next I fire a "near" max loaded cartridge, and again carefully inspect this case. If i don't see anything that looks troublesome, I clean and move on the the last and final proof test using a "Blue Pill" (One to two grains over listed max and if all is good I am finished. Note: I do expect to see some primer flattening and bolt lift stiffness but nothing like the damage you had because I worked up so I could spot a potential problem before It happens.
I don't recommend shooting any load without working up to avoid these kinds of problems irregardless of the cause. Pressure issues will show up many time before reaching max powder charges if we pay attention. One example of this was a friend that couldn't even load the minimum listed load without getting pressure in his 6.4 x 47 L.
After bore scoping the chamber, I found that the chamber had no lead or throat. My guess was that the chamber was cut with a roughing reamer or simply a incorrect finish reamer.
The best advice I can give is to work up "ANY" load and watch for any sign that could tell you you may have a problem. One can't be to careful when it comes to firearm safety.
J E CUSTOM
