HDA3 Hybrid Polymer Brace - HK MP5

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 H&K MP5/SP5. The HDA3 is designed, manufactured, and assembled in the USA.

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 mounting system for the H&K MP5/SP5
• MJF Polymer 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.

Designed, manufactured, and assembled in the USA.