Wait...so will improperly trimmed brass will or will NOT lead to a carbon ring, or make a carbon ring worse? Everyone put your dicks back in your pants, and end this pissing contest.
Not cleaning properly is what leads to a carbon ring and it happens regardless of your brass length because it happens beyond the chamber body.
A carbon ring is beyond the case mouth regardless of length and beyond the chamber, in what is referred to as the freebore and once it gets bad enough will scratch on the bullet just as if you were hitting the lands prematurely.
Letting the chamber get dirty enough to impend on the brass is just a dirty chamber.
I have 20 inch 308, 178eldx @ 2620 with varget easy to find load and one hole @3/8 inch deadly to 500 yards on deer and elk
Also shoot 6.5 CM 135berger at 2800 with 4350.1/2 inch gun. Both are predictable and shoot well. 6.5 if you are patient and willing to only take double lung broadside shots. I have confidence to break a shoulder to get at lungs with the 308. if barrel life is a factor 308 wins.