Early experimental versions of the 9 mm cartridge had a slight bottle neck.
My theory is the step was to accomodate the bottle neck and they didn't bother to change it after going to a straight tapered cartridge.
I don't think my theory is worth a plugged nickel.
On the other hand a tapered reamer would be no great trick, so I don't think it would really be easier to make a stepped chamber.
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