Mossberg 12-Gauge Shell Carrier with Picatinny Rail Mount - 500/590/Shockwave: The two piece design is built to be mounted together as a single unit. Shell holder or rail mount will not install as stand alone units using the included mounting hardware. Unit installs into the factory mounting holes of Mossberg 500 or 590 Series shotgun models with no additional gunsmithing required. Also compatible with Mossberg Shockwave, 535, 835, and 590a1 models.