Carbon on the inside of the neck is a form of neck lubrication. I've been doing a significant amount of testing involving different neck lubes (including none, residual carbon, moly, graphite, HBN, Neolube, and mixtures of a couple of them).
The results have been surprising. As it relates to residual carbon, I've found it to be sort of middle of the road - not the best, not the worst. One thing that was very interesting is that when I ran the uncleaned, residual carbon cases through my 6 BRA (which I run close to max), I got a slightly heavy bolt lift on all cases. Cases with all other types of lube (and lack thereof) cycled normally.