Yankee Hill Machine Co Diamond Series Forearm, 9.290, Midlength, Fits AR-15, Black YHM-9633-DX These unique forearms feature diamond shaped heat vents" an extended top rail for gap-free installation" T marked rails to mark accessory locations" they accept their end caps" and utilize their anti-rotation screws for secure installation. The forearms and barrel nuts are made of 6061-T6 aluminum and hardcoated to military specifications. The jam nut and anti-rotation screws are steel and plated a matte black finish to match.