Product Review – A3 Industries x Haga Defense / HDA3 Hybrid Polymer Stock
The Heckler & Koch MP5 remains one of the most iconic submachine gun platforms ever made, with a silhouette and legacy that spans decades. But when it comes to ergonomics, especially stock design, the platform hasn’t evolved much—until now. The HDA3 Hybrid Polymer Stock, a collaboration between A3 Industries and Haga Defense, brings a much-needed ergonomic and functional upgrade to MP5-pattern firearms.
The standout features of the HDA3 are its adjustable length of pull and adjustable cheek riser—two things rarely found on MP5-compatible stocks. The length of pull is adjustable to seven positions in half-inch increments via an intuitive sliding mechanism, which makes it easy to tailor the fit for various shooting styles, body sizes, or gear loadouts. The cheek riser offers three fixed height positions, allowing shooters to better align with optics while maintaining a comfortable and consistent cheek weld. Check riser adjustments go from zero to quarter to half-inch heights. Adjustable on the fly, it offers meaningful flexibility in the field.
We installed the HDA3 on an MP5 clone, and the results were immediately impressive. Fitment was tight and secure, with zero slop or wobble. The stock slid into place without issue and locked up firmly. The integrated aluminum spine housed within the high-strength polymer body provides solid structural support while keeping weight in check. The polymer components are well-finished, with clean edges and a matte texture that complements the MP5’s aesthetics.
Installation is straightforward—the HDA3 is a true plug-and-play upgrade that mounts using the MP5’s standard rear push pin. The included stock adapter is CNC-machined from 6061 aluminum and finished with a hardcoat anodizing process for enhanced strength and corrosion resistance. The entire assembly fits precisely and locks up solidly, reflecting the level of quality expected from a collaboration between A3 Industries and Haga Defense.
One of the key structural components of the HDA3 is its folding steel hinge, which provides a robust and reliable lockup in both the extended and folded positions. The steel assembly ensures long-term durability and consistent performance. By default, the stock folds to the left and sits at a slight 3.5-degree downward cant to clear the ejection port and controls.
From a functionality standpoint, the adjustable features work as intended, offering tangible ergonomic improvements over the traditional stocks commonly found on the platform. We particularly appreciated how much more natural the sub-gun felt when customized for optic height and preferred shooting position. Once adjusted, everything stayed locked during range sessions, with no signs of drift or loosening.
The HDA3 also includes an integrated QD sling mount, making it easy to adapt to modern sling setups without needing additional hardware or adapters. This kind of detail makes the HDA3 more than just an ergonomic upgrade—it’s a practical one as well.
Visually, the HDA3 brings a modern look to the MP5 while staying true to its original design language. It’s clean, functional, and looks purpose-built—more of a professional enhancement than a flashy add-on.
We’re also excited that A3 Industries produces stocks and pistol brace adapters for a wide array of platforms—not only the MP5. The HDA3 is a sensible upgrade for anyone looking to modernize their legacy pieces or enhance their modern firearms. Its thoughtful features—adjustable length of pull, cheek riser options, solid construction, tight lock-up—make it a standout among buttstock options. It’s not often that a product improves both form and function on a legacy platform like this, but the HDA3 delivers on both. In a world full of clone-correct builds, this stock is for those who actually want to shoot—and shoot well.
A3 Industries/Haga Defense
HDA3 Hybrid Polymer Stock, HK MP5
MSRP: $350
URL: a3industries.com, hagadefense.com
