A shotgun mountable shell holder is provided for holding a single shotgun shell therein. The shell holder includes a rigid body having an external mounting surface provided with a track slidably mounted and frictionally retained on a rail arrangement having at least one rail attached externally of a shotgun. The rigid body also includes an internal shell receiving and engaging structure sized and shaped to slidably receive and frictionally retain the single shotgun shell therein. The track and the rail have mating dovetail-shaped cross sections.