...what do you make of this...

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   Preface: The reload in question has a calculated PSI of just under 40,000 and the headspace was checked prior to and after the issue presented itself. I was extremely grateful that I had a ruptured case extractor with me as well.

I went to the range last week to sight in the Faux L8A5T chambered in 7.62x51mm. I took 80 rounds of ammunition with me using Lapua cases that had been used 3 times maybe 4 prior to that day. Everything was going as planned until round #9,  with the bullet hitting the target 4 inches above the bullseye. It was when I went to cycle another round that I was met with the case head. No biggie I thought and used the case extractor to get the rest of the case out. It went quickly downhill from there. I had to use the case extractor 54 more times that morning and I could tell instantly when I had a rupture/casehead separation afterwards just by looking at the POI from that particular round. I keep a set of 7.62 go and no-go gauges with me and check the headspacing immediately and found nothing wrong and the only conclusion I can come up with is that since the cases were full length resized using 308W dies, the cases had alot more expanding to do to fill the chamber and as a result caused the case failure. But after only 3 or 4 reloads? It doesn't make since. I'm going back in a couple of weeks and taking new cases just to see if this was an isolated incident. 
The stock is getting a treatment of Stand Oil for the time being...