An ejection port door (dust cover) seals the AR’s ejection port to keep dirt and debris out, then flips open automatically when the bolt carrier group cycles. A typical kit includes the door, hinge rod/pin, spring, and retainer clip. It works with mil-spec upper receivers, the bolt carrier group, barrel/extension, and forward assist (if present). Available for AR-15 and AR-10/AR-308—select the correct platform for proper fit and reliable operation.