This disclosure relates to a system and method for storage of a modular cartridge within a cavity formed in a portion of a firearm, wherein the cavity is not the magazine well of the firearm. The disclosure provides for a system for disengagebly retaining such modular cartridges, as well as multiple embodiments of cartridges which may be retained therein. Such embodiments of modular cartridges include: a miniature magazine (mini-mag) which may be used to store ammunition, and which may be operable as a magazine for the firearm; a dry box which may be used to store materials in a manner in which they are protected from the ambient environment; a monopod which may be used to help support a firearm; a flashlight; and a battery charger, which may provide for a usb output.