HDA3 Hybrid Polymer Brace - Sig MPX

The HDA3 is a modular stock and brace system developed through a collaboration between Haga Defense and A3 Industries. Designed to integrate cleanly across a wide range of firearm platforms, the HDA3 delivers strength, adjustability, and durability in a familiar, low-profile package.

This direct-fit configuration mounts securely to the factory interface of the Sig Sauer MPX. The HDA3 is designed, manufactured, and assembled in the USA.

This version is only compatible with the Sig MPX and is not compatible with Sig MCX variants.

Features

Heavy-Duty Folding Hinge

The HDA3 uses a nitride-coated 17-4 stainless steel hinge for a strong, consistent lockup in both the folded and deployed positions. The standard configuration folds to the left at a 3.5-degree cant. Right-folding configurations are planned for a future release.

Hybrid Construction

Metal internal components provide strength where it matters most. The aluminum cheek-riser button and length-adjustment latch interface with a 6061 aluminum extrusion that connects directly to the steel hinge. Exterior components are injection-molded from 30% glass-fiber-reinforced nylon for a durable, lightweight shell.

Stock and Brace Configurations

The stock configuration includes an injection-molded extension with a textured Santoprene TPE buttpad. The brace configuration includes an injection-molded extension designed to accept compatible arm braces.

Tool-Free Adjustment

The HDA3 offers seven length positions in 0.50-inch increments. The cheek riser can be set to three heights: flush, +0.25 inch, or +0.50 inch. Both adjustments can be operated easily with one hand.

Multiple Sling-Mounting Options

The rear of the HDA3 includes an integrated sling loop and two steel QD sockets. Three additional QD mounting points are machined into the aluminum firearm adapter.

Specifications

• Direct-fit adapter for the Sig Sauer MPX
• 6061 hardcoat-anodized aluminum adapter
• Nitride-coated 17-4 stainless steel folding hinge
• Left-side folding with a 3.5-degree cant
• Seven length positions with 0.50" adjustment increments
• 6061 hardcoat-anodized aluminum extrusion
• 7075 aluminum locking latch
• Three-position cheek riser: 0", +0.25", and +0.50"
• Hardcoat-anodized 6061 aluminum cheek-riser button
• 30% glass-fiber-reinforced nylon exterior components
• Santoprene TPE buttpad on stock configuration
• Available in Black, FDE, ODG, and Gray

Length

• Stock configuration: 8.36"–11.36"
• Brace configuration without brace installed: 8.70"–11.70"

Weight

• Stock configuration: 1 lb. 5.6 oz.
• Brace configuration without brace installed: 1 lb. 4.1 oz.

NOTICE: You WILL NOT be able to operate the rear charging handle if the cheek riser is raised. The cheek riser can be quickly moved out of the way but know that pulling back the charging handle all the way will not be possible in the middle or top position.

Designed, manufactured, and assembled in the USA.