Monstrum Mossberg 20-Gauge Shell Carrier with Picatinny Rail Mount - 500/590/Shockwave Black. Built for use with Mossberg 500 and 590 Series shotguns the shell holder accommodates up to seven 20 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). Requires your receiver to be drilled and tapped Specifications for Monstrum Mossberg 20-Gauge Shell Carrier with Picatinny Rail Mount: Manufacturer: Monstrum Color: Black Gun Model: Mossberg Model 590 Shockwave Mossberg Model 590 Mossberg Model 500 Gun Make: Mossberg Hand: Ambidextrous Gun Type: Shotgun Quantity: 1 Condition: New Package Contents: Monstrum Mossberg 20-Gauge Shell Carrier with Picatinny Rail Mount What firearm models does the manufacturer say Monstrum Mossberg 20-Gauge Shell Carrier with Picatinny Rail Mount is compatible with? The manufacturer suggests that Monstrum Mossberg 20-Gauge Shell Carrier with Picatinny Rail Mount is compatible with Mossberg Model 500 Mossberg Model 590 Mossberg Model 590 ShockwaveMonstrum Z84: Mossberg 20-Gauge Shell Carrier with Picatinny Rail Mount - 500/590/Shockwave Black