Stern Defense Mag-AD9 9mm/.40 Mag Adapter for AR-15 Lowers

The Stern Defense Mag-AD9 converts a standard mil-spec AR-15 lower assembly to accept S&W M&P and SIG P320/P250 9mm and .40 S&W magazines. It is machined from aircraft-grade materials and finished for corrosion resistance and durability. The adapter includes a last-round hold-open feature and is designed to work with mil-spec AR-15 lowers when paired with the required bolt and buffer configuration.

Key Specifications

• The adapter is made from 6061-T6 aircraft grade aluminum for strength and weight savings.
• The feed ramp is finished with a 4140 NiB coating to improve feeding durability.
• The exterior finish is a Type III hard coat anodize for surface protection.
• The adapter accepts S&W M&P and SIG P320/P250 magazines in 9mm and .40 S&W.
• The unit is compatible with a standard mil-spec AR-15 lower receiver.
• The design provides a last-round hold-open feature for positive magazine function indication.
• The product requires a Glock-cut bolt to function with the adapter.
• When using a carbine spring the adapter requires a 5.4 ounce or heavier buffer (buffer not included).
• The listed product dimensions are 3.45 x 4.40 x 8.65 inches.

Built to integrate into platform-conversion builds and pistol-caliber-carbine projects, the Mag-AD9 offers a practical path to run common 9mm and .40 S&W magazines in a mil-spec AR lower. Its machined aluminum construction and corrosion-resistant finishes aim to provide consistent function and service life in training, range, or PCC applications while maintaining core AR-15 ergonomics and controls.