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With a good chamber, you are doing pretty good at eliminating possible causes.
This really does not line up with just a larger than bore bullet. Maybe there was something going on with the larger bullet and the new brass. You did have problems pulling a bullet from a loaded round.
I have one other that sounds unlikely. If the round was near max with some space above the powder, it is possible for the bullet to have a glancing blow on a edge on the way in and get pushed deeper into the case. This will push pressure up, but probably not as much as the case indicated. Also, this seems real unlikely based on the hard to pull other round.
Where are we now on "not eliminated" causes. I can only swag 2, are there any others still worth talking about?
Having a film on the ID of a new case combined with a real tight fitting bullet such that the bullet is "glued" in place firmly enough to cause a spike.
A very strange primer/powder interaction with a secondary pressure spike (very low probability)
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OK--I'm still torturing myself over this one, I keep coming back to the very long headspace slack and barrel timing things as the two variables that I generally don't see in any other AR I've had experience with. I don't think the problem is associated with the cartridge dimensions or whether or not the case mouth is wedging to the chamber step--my latest "pet guess" is that there's a timing issue while pressure is still rising in the case to peak pressure--and the case seal to the chamber is overcome and "slams" the cartridge back into the chamber bolt face. That's my latest guess after yesterday's tests in which the first two thirds of the cases showed very little sign of gas leak on the ejected cases--while the higher pressure ones (at approx. 50,000 psi projected) showed extensive signs of gas/carbon leak all the way down the case and accumulating in the extractor groove area.
For the time being, in any new loads I do I will simply work them up to the point I see extensive gas leakage and stop there. In the loads I did yesterday that point was achieved at under 50,000 psi as projected. The excessive gas leakage started at around 45,000 psi projected.
The only actual test data I know of that has actual pressure data associated with it are on Hodgdon's web page--and that's using a 24" test barrel. Who produces one of those??
PS--I had to raise my start pressures in QL to around 6000 psi in order to get my charge weights/velocities close to what I was seeing in my recorded data.