By way of framing my comments I will state that I have a pretty fair understanding of how to cut a chamber in all types of common firearms. In another life I made my living as a machinist and gunsmith.
I have been following the various 'kaboom' threads with some interest and so I decided to take a closer look at the chamber in my 9mm Shield. I was very surprised by what I found. Suffice it to say that the quality of S&W hiring, training, quality control, and management is very suspect in my mind.
It's as though a gorilla with a Dremil was turned loose at S&W and managed to run the grinder up the ramp, over the edge of the chamber an right on into the chamber. The grind is deep enough that I can put the tail of my veneer caliper on it and push against it to get an accurate measurement. It happens to extend into the chamber over 1/8".
I'm also attaching a photo of a cartridge seated in the chamber showing the unsupported portion of the case. I don't have the tools to measure it but it certainly looks excessive to me. Notice the shadowed, dark area that continues on into the chamber.
Next are two photos of my 9c chamber. Much less damage to the chamber here but whoever throated it ran the tool over the chamber edge and slightly into the chamber. It's a passable job but I expect more from S&W. That edge should never be rolled over. All that should be done is to *polish* it. Then the case head support would be excellent as would feeding. I guess 'caveat emptor' applies as much today as it ever has.