Monstrum Side Saddle Shell Holder with Picatinny Rail Mount - Tactical

In this review of the Monstrum Side Saddle Shell Holder with Picatinny Rail Mount, the bottom line is simple: this accessory is for Mossberg owners who want a durable, no-gunsmithing solution to carry extra shells and add optics. It is built specifically for Mossberg 500, 590, and Shockwave Series models and installs using the factory mounting holes with included set screws, so the single biggest reason to buy is the combination of a secure shell saddle and a useful Picatinny rail without modifying the shotgun.

Key Features

• Model compatibility: Designed for Mossberg 500, 590, and Shockwave Series shotguns so buyers can expect a tailored fit on those receivers.
• Shell capacity: Available in two sizes, the 12 gauge saddle holds up to six 12 gauge shells and the 20 gauge saddle holds up to seven 20 gauge shells for ready reloads.
• Rail mounting: The integrated Picatinny/Weaver rail accepts optics and red dots, adding practical sighting options to a shotgun platform.
• Tool-free finish install: Installs with included set screws into factory holes, meaning no additional gunsmithing is required for compatible models.
• Construction: Made from a 100% weave material that presents a solid, finished look and durable surface for field use.
• Legacy fit note: Pre-1997 Mossberg receivers use a different hole pattern and will require 6x48TPI screws (not included), which is important before purchase.

Who It's For

This accessory is best for hunters, range shooters, and home defense users who own a compatible Mossberg 500, 590, or Shockwave and want quick access to extra shells plus the ability to mount a compact optic. It suits users who prefer bolt-on accessories that need minimal tools and time to fit.

Those who should look elsewhere include owners of Mossberg 500 Bantam, Combo, Flex, or any model where the forend extends past the receiver when pumped, because the unit is not compatible with those variants. Also, buyers of very early pre-1997 Mossbergs should be prepared to source the correct screws separately.

Pros & Cons

Pros

• Tailored fit and finish for select Mossberg models provides reliable mounting without gunsmithing.
• Good shell capacity options for 12 and 20 gauge shooters to carry multiple rounds on the receiver.
• Integrated Picatinny rail adds flexibility for optics or red dots on the shotgun.
• Customers report solid build quality and a clean appearance when installed.

Cons

• Some users report the shell holder can interfere with forend travel on certain setups, which may affect operation.
• Pre-1997 models require specific 6x48TPI screws that are not included, adding a small extra step for older guns.

Specifications

Brand	Monstrum
Compatible models	Mossberg 500, 590, Shockwave Series
Not compatible with	500 Bantam, 500 Combo, 500 Flex, models with extended pump travel
Shell capacity	12 gauge: up to 6 shells; 20 gauge: up to 7 shells
Mounting	Installs via included set screws into factory mounting holes
Rail standard	Picatinny / Weaver
Pre-1997 requirement	Requires 6x48TPI screws (not included) for older receivers

Our Verdict

For owners of compatible Mossberg 500, 590, and Shockwave shotguns this side saddle with Picatinny rail is a practical upgrade that combines extra shell capacity and an optics platform without permanent modification. It offers strong build quality and useful features at a modest cost, though buyers should confirm model compatibility and consider the pre-1997 screw requirement if applicable.

Frequently Asked Questions

Will this fit my Mossberg 500?
If your model is a Mossberg 500, 590, or Shockwave Series it is designed to fit, but it does not fit Bantam, Combo, Flex, or models where the pump extends past the receiver.

Do I need tools or screws to install?
The kit installs via included set screws into the factory mounting holes so no gunsmithing is required for compatible models; pre-1997 receivers will need 6x48TPI screws not included.

Can I mount an optic on the rail?
Yes, the integrated Picatinny/Weaver rail accepts standard compact scopes, red dots, and similar optics.