If the round is still stuck in the chamber, keeping your head away from the muzzle, slam the rifle on a hard surface, butt down and the bolt carrier will release and extract the round 99% of the time...
If the chamber is empty now, and I know you said you cleaned the chamber with a chamber brush, but not how you accomplished that....I, as well as others that have shot high round count competitions, have run into the same problem in the past...It's usually due to excessive powder debris build up in the chamber throat and normal cleaning with a chamber brush won't touch it...
Install you chamber brush in a section of cleaning rod that is chucked into a cordless drill motor...Apply a few drops of good lubricant and spin the brush in the chamber for 20-30 seconds and then patch out the debris...It should cure your problem...It's now part of my normal cleaning routine and haven't had a similar problem since...