Built to work and priced to own, the Sig Sauer M400 Sig ROMEO5 5.56 NATO 16″ brings dependable 5.56 NATO performance to the AR-15 platform without the premium cost of boutique manufacturers or the compromises of budget builds.

The Sig Sauer M400 Sig ROMEO5 5.56 NATO 16″ is a semi-automatic AR-15 rifle chambered in 5.56 NATO, built around direct impingement operation for reliable cycling on the AR-15 platform.

Consistent Cycling: The direct impingement system delivers consistent cycling across the standard 5.56 NATO ammunition spectrum without requiring frequent cleaning or special maintenance.

Versatile Configuration: With a 16″ barrel and standard AR-15 ergonomics, this rifle fits the broadest possible range of users and use cases without compromise in any single area.

Upgrade-Friendly Platform: Everything on this AR-15 is standard — stocks, grips, handguards, triggers, and optic mounts can all be upgraded without specialized tools or gunsmithing knowledge.

Weather-Resistant Finish: The Type III Hardcoat Anodized finish provides lasting protection against corrosion and everyday wear — reliable function from the first range trip to the last.

Caliber: 5.56 NATO
Barrel Length: 16″
Gas System: Direct Impingement
Handguard: Free-Float
Muzzle Threading: 1/2×28
Magazine Capacity: 30 Round
Finish: Type III Hardcoat Anodized
Origin: Made in the USA

Works with standard AR-15 lower receivers and accepts STANAG 5.56 NATO magazines. Designed for mil-spec carbine buffer systems and compatible with the full ecosystem of AR-15 parts and accessories.

Receivers are machined from 7075 aluminum and finished with Type III Hardcoat Anodized, providing lasting corrosion and wear resistance under regular use and varied environmental conditions.

Covers the core use cases — home defense, range work, and recreational shooting — without specializing in any single role, making it a reliable all-purpose AR-15.

SIG SAUER is trusted by the U.S. military and law enforcement agencies worldwide. Their AR-15 lineup delivers the same premium quality and engineering that earned SIG that trust. Made in the USA.