Add another vote for out of battery firing. I would be taking a very close look at the Buck Mark and trying with all my might to get that hammer to fall with the slide out of battery to attempt to "re-create" the failure (but without live ammo) Because THAT is a serious problem and if it's something that can happen again, it needs to be addressed.
I would have no solid reason to blame the ammo with the evidence that's been provided.
Attention Brass rats and other reloaders: I really need .327 Federal Magnum brass, no lot size too small. Tell me what caliber you need and I'll see what I have to swap. PM me and we'll discuss.