Upper receivers are components in a firearm, specifically in AR-15 style rifles. They house the bolt carrier group and are crucial for the cycling of the weapon. The upper receiver connects to the lower receiver, allowing for the assembly of various parts, such as the barrel and handguard. These receivers come in different configurations, including flat-top and A2 styles, which affect the mounting of optics and sights. Made from materials like aluminum, upper receivers are designed for durability and precision. They play a vital role in the overall performance and functionality of the firearm.