really weird chambers

[Waschbaer]

i'm sure several of you have noticed this but... In the blueprints for original parabellum barrels (9mm) you'll notice a small shoulder in the middle of the chamber, I didn't encounter this in my friend's wwii parabellum, but my 1915 sure has it. My empties come out with an unmistakable bottlenecked effect. What on earth is the purpose of this shoulder, if any???