That's unburned powder except that it got warm enough for the graphite coating to burn off and sometimes gets warm enough to melt in with other such lumps, distorting the shape. I'm not sure if the yellow is due to smoke staining or just the fact the nitrocellulose was made by nitrating wood pulp instead of pure, virgin cotton, but it's what you usually find in these situations.
The reason you got those is the same reason you have the unburned dark flakes. The cartridge is running at a pressure too low to burn thoroughly or efficiently. If you go to reload these yourself, using a faster burn rate powder will clear it up.
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