Magazine extensions have an extension body having a floor panel, a right side wall, and an opposed left side wall, and a front wall, the right and left side walls each including linear engagement facilities each configured to engage the floor plate attachment facility, the extension body defining a chamber between the right and left side walls and rear of the front wall, and open in an upward direction, the linear engagement facilities being operable to constrain movement of the extension body between an attached condition in which the chamber is registered with the internal passage, and a detached condition, a door slidably and removably connected to the extension body and having an upper door edge, the door movable between an attached condition in which the door spans between front or rear portions of the right and left side walls, and a detached condition.