What happened to this brass?

My first impression is that this must have been a very light load, since you're not getting obturation (a good seal around the case), and it's leaving lots of unburnt power in the chambers; this also allows the gas from the powder that DOES burn to leak around the edges of the case mouth, darkening it the way you show. What's the load you were using for these (powder and weight)?

+1 on SDC.

Work up a load with a magnum primer, might just get a cleaner burn. I always use a mag for 38 & 357. Bullseye or 2400, dont matter.

i have that as the starting load for a 125gr. 6.8gr as max. Needs more powder.

Please reduce the size of your pictures. This page will take until Tuesday to load for a dial up member.
What black part? The wee ring around the rim? That'd be lube gunk or caked carbon. If the load was too light, the whole case would have carbon on it. Looks like the cases bounced on stones or whatever that cut through the nickel plating though. Not a big deal, but the rest of the plating will likely flake off.

That actually looks pretty normal to me, and I load gobs of 38 and .357.
Then again, are you trimming all of them to the same length? If not your crimp will be different from one to the next which might cause some varying burn efficiency. What inspired me to mention that was the unburned powder on the outside of the case. Also could be too light a charge as already mentioned.
Just keep working the load and see if things change any.

I tried AA#5 for .45acp and it worked great. Then I thought I'd try #5 for .38spl with my starting loads I had the same result as you, as I worked up the loads AA#5 burned clean at the upper end of the load data.
Good case neck tension and a heavy crimp helps, the unburnt powder will tie up the action.

6.1gr of #5 pushing a 125 gr HP 38sp.

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6.1gr of AA#5 is close to what I use with a 158gr bullet.
http://www.accuratepowder.com/wp-content/uploads/2011/01/accurate_load_data_3.5.pdf

What are the tool marks about an eighth of an inch from the case mouth?

The tool marks are a case cannelure. They're to help prevent bullet setback.

SDC said:

My first impression is that this must have been a very light load, since you're not getting obturation (a good seal around the case), and it's leaving lots of unburnt power in the chambers; this also allows the gas from the powder that DOES burn to leak around the edges of the case mouth, darkening it the way you show. What's the load you were using for these (powder and weight)?

Click to expand...

Walkalong said:

Agreed.

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.........same here.

RidgwayCO said:

The tool marks are a case cannelure. They're to help prevent bullet setback.

Click to expand...

Or to identify certain loads or bullet weights within a particular manufacturer's line of ammo.

I don't shoot a revolver yet, but I have 38 and 357 brass sorted from my range scrounging. The thing that puzzled me was some brass has it, others not.