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.