I am more than convinced that it is an ammo batch problem Dan. Ignore me if I've said this before but what about trying some brass cased 762x54. I appreciate that steel is strong and tough (?) but there's steel - and there's steel. And you can be sure that the one-way-trip steel your cases are made of ain't gonna be good, believe me.
I appreciate that there's brass and there's brass too, but brass is very pliable and elastic. But the metallurgists here such as JM, Breaky and others that while the steel cases blow out and re-form slightly at the neck (in the recut chamber) it's supported while it does so. At the UN-supported extractor way, the steel blows. But not all steels......... some is as good as gold. You gotta find the batches that ain't going to blow