Stern Defense Mag-AD9 AR-15 Adapter for M&P & P320 Mags (9mm/.40)

The Stern Defense Mag-AD9 is an adapter designed to allow standard mil-spec AR-15 lowers to accept S&W M&P and SIG P320/P250 pistol magazines in 9mm and .40 S&W. Constructed from aircraft-grade aluminum and finished with a Type III hard coat anodize, the adapter installs in minutes and retains standard AR-15 lower ergonomics. The design includes a 4140 NiB-coated feed ramp and supports the adapter’s last-round hold-open functionality when used with compatible magazines and a Glock-cut bolt. Intended for builders and shooters converting an AR lower for pistol-caliber magazines and low-recoil rifle configurations.

Key Specifications

• Constructed from 6061-T6 aircraft-grade aluminum.
• Finished with Type III hard coat anodize.
• Includes a 4140 NiB-coated feed ramp.
• Accepts S&W M&P and SIG P320/P250 magazines for 9mm and .40 S&W.
• Designed to fit mil-spec AR-15 lower receivers.
• Requires a Glock-cut bolt.
• Recommends a 5.4 ounce buffer or heavier when using a carbine spring.
• Overall dimensions listed as 3.45 x 4.40 x 8.65 inches.
• Manufactured in the United States.

Whether you are building a pistol-caliber carbine for economical range time or converting an existing AR platform to run S&W M&P or SIG P320/P250 magazines, the Mag-AD9 provides a robust, aircraft-grade solution. The adapter’s durable materials, proven feed ramp finish, and straightforward installation make it a practical component for builders and shooters seeking reliable magazine compatibility in 9mm and .40 S&W configurations.