Abstract: A magazine insert may include a housing having an opening. A follower may be mounted in the housing for pivot action relative to the housing, with the follower being exposed through the opening in the housing. A biasing member may be provided in the housing to influence the follower toward the opening. When installed in the well of a firearm, the magazine insert may prevent contaminants from entering the firearm, and may provide an active retention feature by which a bolt carrier group can be retained in a retracted position without having to manually actuate a bolt catch.