Yankee Hill Machine Co Dust Cover, 300 Blackout, Fits AR Rifles, Black, Includes: Dust Cover, Rod, and Retaining Clip Spring YHM-111-300 YHM caliber marked dust covers for those who may end up changing the caliber of their top end/upper receiver. Assemblies Include the following: Caliber Marked Dust Cover" Rod and Retaining Clip" Spring.