Abstract: A shell catcher apparatus that is supported from a connection on a removable ammunition magazine and from there extends a receptacle shaped to catch and retain the spent shells as ejected from an auto-loading firearm. Supported on the magazine, the primary feature of avoiding direct mechanical connection to the firearm frame is achieved. The shell catcher to magazine attachment component may be a clamping bracket or alternatively a modified replacement of an OEM part such as a sole plate or outer case of the magazine. The receptacle component of the shell catcher is shaped in a manner to extend its opening to the location where the spent shells are ejected from the firearm action. By the nature of its function, any shell catcher design obstructs access to the action of the firearm. By mounting the shell catcher on the magazine they become unitized and are both removed when the magazine release button is operated.