Carbon ring?

[Webster]

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1st picture below is with a piece of brass in the chamber, showing the end of the case mouth, the end of the chamber, and beginning of the free-bore, before cleaning/removing the carbon build up/ring:

This next picture below is after some cleaning of the area, again with the case in place:

This 3rd picture is the same, but with the case removed:

At this point the chamber-end is clean & carbon free, with a square transition up to the free-bore, no longer a rounded transition formed from carbon layer (aka: carbon ring).
The area in the picture is like 85% cleaned & carbon free. What is left to be cleaned in the picture is the remaining heavily carbon layered beginning of the free-bore, where the cracking of the carbon layer itself still remains, that can be easily seen in the photo (and of course the rest of the free-bore/throat and bore not in the picture).

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Great images. Good job.