I just ordered a .356 bullet sizing die.
I still would like to know definitively why there is a malfunction--I get obsessed about getting to the root cause of these things--among other things if I don't there will always be that "might be skating on thin ice" doubt even when things are working fine (I suspect you already know that feeling having moved down to the .356 die
). So far, P Flados observation of the case mouth making it past the step-down in the chamber and locking the crimp seems the best explanation, even though the OD in my .357 loaded cases appear to exceed slightly that for .355 loaded bullets.
At first I thought that asymmetric compression of the case rim was due to the case head being slammed back into the bolt face. Now I'm confident it's more likely that it is simply a pathway of least resistance as pressure in the case ramps up--just like the ejector--and thus expanded outward with less resistance initially (the extractor readily shows the outward bending of the claw) while the rest of the case is contained within the bolt fcae wall.