I had a partial case head separation using a 1.8oz buffer

[LRRPF52]

grayfox (and Klem) I understand all of what you say. Clearly there is not enough statistical data regarding this failure to be outright proven, but note that there is not enough data to prove the otherwise or any other failure as well. All of us can only speculate and give our subjective opinions here (which to me is the point...and fun!). It goes both ways with point and counterpoint on a technical debate. As an engineer I am just working with the "data", albeit anecdotal, that's just available in the moment. And of course we should have other failure models on the table here and keep them open too.

And just to be clear on my position/current opinion, I definitely think OP was running a hot load. He was above Hornady's max load with a Hornady bullet. His casing could have easily been Hornady or Norma and still fail.

Also I agree about Lapua being too expensive to be worth it.

The stated load in the OP is not hot, given a normal chamber and the dimensions he has since posted that his chamber will allow.

Hornady’s load data bundles the 120gr GMX with the 120gr ELD-M and 123gr ELD-M, with the 120gr GMX being the limiting bullet with totally different start pressure considerations.

120gr GMX is a solid monolith, whereas 120gr and 123gr ELD-Ms are cup and core, much easier to force into the rifling.

Even if you loaded an additional grain of powder under a harder jacket bullet...as with Alexander Arms published 123gr Scenar data (30.7gr of AA2520 on top of the longer Scenar), you would not see this type of case head failure in a SAAMI spec chamber, and would still be within or at the MAP.

We noticed this about the Hornady data years ago, where GMXs are lumped-in within cup and core bullets. When we reached out to Hornady about the matter, it came down to the additional page count that it would add to their comprehensive cartridge load data books, which are already Biblical in size. They wouldn’t just have to include GMX data for 6.5 Grendel, but every other bore diameter/cartridge that has GMX bullets available for them.

If you are looking for a sanity-check in this regard, or are like me-very skeptical until I see sound evidence, look no further than Hornady’s data for 129gr SP, Interbond, and SST.

You will notice that since there isn’t a GMX in that weight class, the maximum loads for 129gr cup and core bullets are actually higher than their 120-123gr class.

In a properly-cut chamber, you typically have to exceed the brass yield strength to see case head separations or failures on initial firing.