Extra step in chamber: Has anyone seen this before?

[mrerick]

I am afraid that that the stepped chamber is part of the Parabellum's design.

It's a desirable feature involved in helping achieve a more effective gas seal for the 9mm straight walled cartridge.

It was expensive to manufacture, and later Lugers sometimes eliminated this feature.

Hard to say what the result of your modifications will be...

The cycling and cartridge feeding of a Luger is a achieved through a complex balancing act. The magazine is the first place to look when experiencing feeding problems. Replacement with a modern MecGar mag is a wise first step.

Sometimes the wrong recoil spring has been installed in a Luger, which changes the cycle timing. While your 1920 commercial 9mm gun used a 18-19 coil recoil spring of medium pressure, later Mauser Lugers used one with 20-21 coils and more pressure.

Sometimes Lugers can be ammo sensitive. Always use standard velocity 9mm 115gn or 124gn jacketed ammo.