The carbon ring will appear more easily if your neck is trimmed short in relation to your chamber, what you first want to do is to get a chamber gauge and measure the actual max chamber length, and trim to max 0.10 or less below chamber max length. To allow for some brass growth, needless to say you have to monitor brass flow carefully if doing this, as going too low will give serious pressure problems.
As following book trim lengths might not be optimal in your chamber.
This will help the brass seal the chamber faster and reduce the amount of carbon
deposits that will flow into the unsealed chamber clearances, the effect usually gets worse the slower the powder burns.
If brass is new and too short for your chamber it might need a firing or two to grow to optimal length,then the only thing that helps until your brass is at length is a more clean burning powder.
If your not really concerned about maximum velocity, i would highly advice you to look into Vihtavuori N100 series powder, the 100 series are generally very clean burning and easy on the throat, pretty temperature stable, with good lot to lot consistency, and decent velocity. For the 6,5 Grendel with 123 grain bullets, N133 and N135 should be optimal. If shooting heavier bullets N140 is worth a try.
This should help reduce carbon fouling.
And if you need max velocity the 530 and 540 will give that, but is harder on the throat and temperature sensitive as other double base powders.
By the way if you don't mind to read some, here is a rather informative topic from yesterday about this, with advice from some people that knows more then me about this subject:
Reloading video and brass growth Just read the post all through and dont pay much attention to the video.
