I haven't seen that in pistols but some HK rifles have fluted chambers for more reliable extraction and the fired cases look like that. My OCD about brass and reloading have prevented me even shooting one much less buying an HK rifle despite their stellar reputation.
Probably fired from an HK Submachinegun.
You all beat me to it. My PTR-91 does the same thing.That is carbon from a fluted chamber. Does not hurt a thing, for reloading it cleans right off.
My Cetme does that to 7.62 brass.
HK's have fluted chambers.
HK MP5, P7's...... probably others.
OP, the MP-5 and HK-91/G-3 use a roller-locked delayed blowback action, and their chambers are fluted to facilitate reliable extraction through that process. All of these weapons and their clones will do that to the brass.