D-S,
I'm wondering if your hotter load are trying to open the bolt too soon, leaving the case head somewhat un-supported during cycling operations.
To check this, clean everything up swab the chamber of debris and oil, you do not want oil in the chamber when you fire this. If you think it's safe to shoot, using a pristine case, shoot it again, one more time! But take the bolt apart first, you don't want the extractor or ejector contaminating the results. Measure the case for the bulge Before and After the test. Shoot the weapon single shot , BUT with the mag in the weapon. Single load it manually into the chamber!
Remember to shoot this with an unloaded Magazine in the weapon (important).
Bang!
First, did the BCG hold open? Second you better have a larger type Cleaning Rod with say a 375 Jag and then lightly bang the case out from the chamber. Thirdly, measure and examine the fired case. If no bulge can be observed or measured then a slower powder is in order. But no matter what, this should point out the deficiencies you are looking for. Like, because you used a pristine case, are the scratches gone?
Now, if you think this is safe shoot it, then return and report..
..t