An upper receiver for a firearm. The firearm includes a barrel extending forward of the upper receiver and a buttstock extending rearward of the upper receiver. The upper receiver includes forward gas escape ports through a frame of an ejection port through which spent casings are ejected. The upper receiver further includes a trough formed on an inside surface of the frame. The trough collects gases in the chamber and distributes the gases to the gas escape ports. The gas escape ports are covered by an ejection port door when a shell is not being ejected.