"Chicago Mod", fact or fiction?

I believe that they are restricted to pistols of DAO persuasion, if that's what you mean. Sigarms will swap out a few parts to make it so.
 
Just learned something new. This is very interesting, as all auto pistols feature this design rib. I think it may have more to do with Sigs older style, pinned-in breechblock, as the gun wears down with use. Picture the breechface shifting downward slightly, as if it were pivoting on the axis of the breechblock retaining pin. This would expose that sharp 90 degree corner in the pictures to the cartirdge primer. I can't think of why a partially loaded mag, or slamming into the frame, would exacerbate the problem?
 
The key to the incident was that the mag was partially loaded and when slammed into the gun, the top round nosedived causing more of an exposure of the primer. The corner of the rib on the breachblock then hit the primer.

New stainless slide Sigs have that corner rounded, as well as the carbon steel slide ones after 1995. Again, it's something I wouldn't worry about and I think it's an isolated case that Sig "fixed" anyway.
 
I think there is an old thread with the complete story under SIG safety warning or something like that. If you have a S&W 3rd gen and look at the underside of the slide where it strips the round you will see that the front edge is beveled to prevent a corner from catching a primer should a round nosedive in the magazine.
 
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