Yankee Hill Machine 110: The Yankee Hill Machine Flat Top Upper Receiver is made from 7075-T6 aluminum (Forged) and has a hardcoat anodized matte black finish. It features M4 Style feed ramps and \"T\" Markings and the inside is coated with dry film lube. It accepts standard upper receiver parts (dust covers, forward assists, etc.) and Mil-Spec lower receivers. It will fit and function with your standard lower receiver.