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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.
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I agree, I do not think the issue is associated with the larger dimeter bullet and the bore measurements alone. It might be as simple as wimpy brass--I have some winchester and hornady brass coming in and I want to carefully compare all the alternatives.
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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.
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I thought about this too--while out live firing I made a point of double-checking the cartridges--and ejected some without firing after cycling--I saw no notable signs of abrasion or bullet movement.
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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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Yesterday's test with the .355 bullets were interesting in that many of the cases fired and came through clean--but I reached a point eventually where gas leakage down the side of the case and to the base started happening--I stopped at the top charge weight after I saw the tell-tale gathering of carbon deposit in the area of the extractor claw on the rim. Why? I don't know. I tend to think it's not associated with head spacing properly off of the case-mouth per sae--but there is a lot of room for movement between the case mouth step in the chamber and the bolt breech face. Also, as I've said, the use of carbine gas system producing an unusually long dwell time length in my 20" barrel is definitely something I don't understand--at least from the point of view for timing. I guess I should give Faxon a call and try to get some dope on that.