It's a dust cover. It keeps debris out of the action while it's closed and can act as a bolt hold-open device if you pull the charging handle to the rear and shut the trap door.WildRice said:What is the purpose of the plastic trap door in the inside front of the ejection port? It has a spring loaded hinge and sliding inner part. Does it reciprocate during the firing cycle? It seems to stay in the forward/down position.