Also called a dust cover, and it does exactly what the name says: keeps dirt out of your upper when the rifle isn't in use. This is a complete mil-spec AR-15 trapdoor kit, so you get the door assembly, the spring, the rod, and the C-clip all together. The rod is machined from stainless steel and finished in black nitride, which is a step up from the plain steel rods that tend to rust. It installs on any standard AR-15 upper.