Monstrum Side Mounted 12 Gauge Shell Holder Picatinny Rail Mount for Mossberg 500/590/Shockwave Series Shotguns Black. Built for use with Mossberg 500 and 590 Series shotguns the shell holder accommodates up to six 12 gauge shotgun shells. The ambidextrous design allows for installation on either side of the stock for shooters of all orientations. 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. Please note that pre-1997 Mossberg shotguns were manufactured with a different mounting hole pattern and will require 6x48TPI screws (not included). Specifications for Monstrum Side Mounted 12 Gauge Shell Holder: Manufacturer: Monstrum Color: Black Number of Rounds: 6 Hand: Ambidextrous Gun Model: Mossberg Model 500 Condition: New Quantity: 1 Gauge: 12 Mount Type: Picatinny Attachment Type: Picatinny Package Contents: Monstrum Side Mounted 12 Gauge Shell Holder Monstrum Z83: Side Mounted 12 Gauge Shell Holder Picatinny Rail Mount for Mossberg 500/590/Shockwave Series Shotguns Black