[mrerick]

The brass is "springy" and slightly malleable because it's job is to be pressed against the chamber wall and seal during firing the round.

The darkened area on one side of the brass is blow-by (blow-back) that is occurring because the brass is not sealing against the chamber wall evenly.

The Luger chamber (except for the very end of Mauser production in late 1941 and 1942) was made with an offset obturation ring - stepped chamber. The circular mark around the brass you see is from that step in the chamber.

Here's the drawing associated with 9mm cartridges and Luger's patent on this:

Headspace is associated with the mouth to base cartridge measurement. The mouth rests on the right most step just prior to the leade into the barrel rifling. The breech face must be very close (thousandths of an inch) to the case base when the pistol is in battery.

You'll also notice that the entire chamber is very slightly tapered, including the area between the obturation ring and the right end of the chamber.

You may be dealing with damage or wear or dirt clogging an area around or in front of or just behind the oblation ring.

I've attached a copy of Luger's German patent number DE237192A from 1910.