What causes this?

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I went to the range yesterday and found a bunch of 9mm brass that looked like this. What causes this? Oversized chamber? The brass also feels scored just next to the black stripes. I tossed a few in the tumbler and this picture is after an hour in corn cob media.
 
Hmm. Interesting.. What type of gun has the fluted chamber?
 
It wasn't from any of my pistols. It was brass I found in the bucket at the range.
 
Fluted chambers are a technology usually associated with H&K rifles (*91) and pistols (P7 series). In both case Heckler (you suck) & Koch (and we hate you) has timed their gas-operated arms to begin extraction well before nature intended, so the brass is still stuck quite hard to the chamber walls by high pressure.

Rather than taking a hint and delaying extraction, they fluted the chambers so that the hot gas would help unstick the brass a bit early. The 91 series rifles still tend to tear rims up pretty bad.
 
Cool. MP5 would make sense since there was boatload of brass there. I grabbed a bunch to see if they would clean up to reload, but they don't get "pretty" enough for me. I have enough 9mm brass that I don't need to load them. Thanks for the info. I enjoy learning new things every day.
 
edwardware said:
Fluted chambers are a technology usually associated with H&K rifles (*91) and pistols (P7 series). In both case Heckler (you suck) & Koch (and we hate you) has timed their gas-operated arms to begin extraction well before nature intended, so the brass is still stuck quite hard to the chamber walls by high pressure.

Rather than taking a hint and delaying extraction, they fluted the chambers so that the hot gas would help unstick the brass a bit early. The 91 series rifles still tend to tear rims up pretty bad.
Click to expand...

Exactly!!!
One of the reasons I don't own a HK 91 or P7 pistol.

For H&K, it suited their needs and it didn't suit mine.
 
Our Mp5 and Mp5k sub guns work with boring reliability on full auto, burst and semi auto modes... and have been doing so for the past 20+ years with only spring replacement for major maintenance.

I’d say H&K had their fluted-chamber system dialed in pretty darn well :thumbup:.

Stay safe.
 
I was fixing to say that looks just like the brass the MP5K Full Auto Rentals throw at the range. I think they mostly run European or Israeli NATO Spec Ammo through them so it could be a combo of fluted chambers and pressure.
 
edwardware said:
Fluted chambers are a technology usually associated with H&K rifles (*91) and pistols (P7 series). In both case Heckler (you suck) & Koch (and we hate you) has timed their gas-operated arms to begin extraction well before nature intended, so the brass is still stuck quite hard to the chamber walls by high pressure.

Rather than taking a hint and delaying extraction, they fluted the chambers so that the hot gas would help unstick the brass a bit early. The 91 series rifles still tend to tear rims up pretty bad.
Click to expand...

It's not all about timing, it's about having no primary extraction. There's no twisting, turning, or tilting to break the brass free from the chamber walls.

Also they aren't gas operated. They are delayed roller locked. There's no gas system. On the rifles anyway.
 
Jim Watson has the best idea. Brass does not *need* to be clean to work. I reloaded for a number of years with tarnished brass that I wiped down with a rag. It shot fine. I would keep that brass for use somewhere that I was likely to lose the brass, or just use as it is now. After a couple cleanings you will never know it was stained.
 
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