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I was picking up my 45 acp brass at the range when I found several of these 9mm spent rounds.
They weren't shot by me.

There were many of them. They all had the same scorch marks and the same severely dented mouth.
Primers look normal.

If these came from my gun I would be concerned.

Any idea what might cause this?

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Ruger .44 Carbine, Security-Six, Service-Six, Mini-14, .30 Carbine Blackhawk
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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.
 

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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.

Bruce
Probably fired from an HK Submachinegun.
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.
You all beat me to it. My PTR-91 does the same thing.

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.
 

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The fluted chamber causes the carbon "stripes" not harmful .
The dented case mouth can even happen when a extracted shell hits the concrete floor of the shooting range .
A buddy thought his gun was "defective" but I threw down a blanket and after fireing a few rounds ... no dented mouth's ... they were just hitting the concrete range floor and dinging the mouth .
Gary
 

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Except for the stripes, they resemble what I see when starting out the development of a new load where none of the reloading manuals have exact data for a new bullet I'm using (something that has happened a few times in the last few years.)

Starting out at 10% under the published starting point for the closest thing I can find in a manual, the round will often not have enough pressure to expand and seal itself in the chamber completely, resulting in carbon getting into the chamber and on the cartridge. The low power load will also often have problems cycling a slide, where the cartridge will get extracted, but not fully ejected, and the mouth of the thing will get slammed against the lip of the barrel as the slide closes on it, creating the dent.

Given the indications of a fluted chamber, I would guess someone is testing a subsonic round they loaded up for their HK.
 

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With my Guardian 32s, the entire front third of the case is evenly scorched. It has begun to extract before the gas has left the barrel.
 

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My MP5K H&K does brass that way ..more so when it is really really dirty ..likely the owner does not know the proper maintenance procedures as he is way past a chamber servicing ..he will shortly as it starts eating extractors and ejectors especially ..pick up the brass in 99% of the cases as the civilian 9mm ammo does not have enough pressure to "fireform" the ridges and the true machine gun military ammo brass case which have higher pressure also have thicker cases so they as well will not fire-for the ribs
Bear
 
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