Pics of Carbon Ring?

[DSheetz]

When the powder burns it doesn't all burn at the same rate or completely when it reaches this area . As the temp of the barrel is cold compared to the burning powder it causes the incompletely burned gas and powder to condense in this area under pressure and coat it with carbon kind of like powder coat painting .

 

[rammac]

I don't see that at all considering, in most cartridges, somewhere around half or more of the powder charge follows the bullet down the bore, it doesn't all burn by the time the bullet engraves in to the lands.