A cover for a firearm is disclosed. The firearm cover is shaped to complement the bottom of an upper portion of a two piece receiver. The firearm cover may be pinned to the bottom of an AR-15® upper receiver and may cover a bottom opening and/or a rear opening when in place. When secured in place, the firearm cover may capture a bolt carrier within an inner cavity of the upper receiver. Additionally, the firearm cover will protect the inner cavity, keeping dust, debris, and moisture out of the upper receiver.