Every 2,000 rounds or so, you need to remove the flash hider or muzzle brake on your 5.45 AR-15. Use a 19mm wrench and steady pressure. Then put it forward end down and take a look.
That's not supposed to be there! Light tapping with nearly any tool will show you that is indeed built up carbon. Do note that I've never seen this actually cause a problem in 27k rounds. I assume the rounds exiting the flash hider self regulate the carbon wall from obstructing the bore but might as well spend 10 minutes chipping this away and then soaking in my favorite penetrating oil and repeating the chipping process. You don't have to get it perfect; just knock out most of the wall of carbon with a flat tip screwdriver or any other tool you have laying around.