“It’s usually a muzzle brake to reduce the recoil. Not much point in trying to silence a round delivered at a supersonic speed.”
Quick detach over muzzle brake. Also reduces muzzle flash.
https://www.youtube.com/watch?v=vEgBvKOgVuM
Video of three.
https://www.youtube.com/watch?v=6cK9WQeNqN4