I got to thinking about cause and effect and here is what I know. All of my brass is Weatherby brand and properly stamped to the caliber. What I don't know is how many lots of brass might be mixed in together, and how old some of it might be. I got some of my brass from a brother-in-law that might be pretty old (and I think I can now safely assume reloaded a few times).
What else you don't know is how the mixed brass has been sized.
If it's been fired a few times & sized incorrectly with the shoulder pushed back a few time & fired, the case has very likely stretched & weakened ahead of the belt.
The advice to get new brass & start over is the only solution & then when re-sizing, just size down enough to allow the case to chamber & do not push the shoulder back even though it's a belted case.
After multiple firings, you may also want/need to eventually buy a collet sizing die to further size the case body just ahead of the belt more than is possible with a standard sizing die.
Collet Sizing Die for Belted Brass MM
