FN 36421 AR-155.56x45mm NATO 16" Carbine Length Gas System, Black Phosphate Cold Hammer Forged Chrome Lined

The FN 36421 is a 5.56x45mm NATO AR-15 carbine barrel built on over a century of FN's barrel-making expertise. This 16-inch cold hammer-forged barrel pairs battle-proven materials with meticulous manufacturing to deliver the accuracy and longevity serious shooters expect. Whether you're building a duty rifle, a precision carbine, or upgrading an existing platform, FN's hammer-forged barrels have earned their reputation through performance in demanding environments.

Cold hammer forging is the foundation of FN's approach—it aligns the grain structure of the steel and eliminates the voids and stresses that can compromise accuracy and lifespan. The barrel is manufactured from the same high-grade Chrome Moly Vanadium steel FN uses in its machine gun barrels, proven to withstand sustained punishment. After forging, every barrel undergoes Magnetic Particle inspection to catch micro-stresses or defects before they leave the factory, ensuring consistency from rifle to rifle.

Durability starts with the chrome-lined bore and chamber, which resist corrosion and heat erosion—a major advantage for shooters who run suppressed, shoot frequently, or operate in harsh conditions. The matte black Manganese Phosphate finish on the exterior resists external rust and oxidation. M4-style feed ramps and the barrel extension provide reliable feeding and extraction across a wide range of ammunition and shooting scenarios. The barrel is dimensioned at .750 at the gas block, compatible with standard AR-15 gas systems and components.

Available here in the 16-inch carbine-length configuration, this is a direct bolt-in replacement for standard AR-15 builds or a foundation for a new build. FN barrels represent precision manufacturing without the hype—just consistent, reliable performance.

Key Features

• Cold hammer-forged from Chrome Moly Vanadium steel for strength and durability
• Chrome-lined bore and chamber for corrosion and heat resistance, extended barrel life
• Magnetic Particle tested to eliminate micro-stresses and defects
• M4-style feed ramps and barrel extension for reliable feeding and extraction
• Matte black Manganese Phosphate finish resists external corrosion
• .750 gas block diameter for standard AR-15 compatibility
• 16-inch carbine-length gas system

Specifications

Frequently Asked Questions

Will this barrel work with my current AR-15 upper receiver?

Yes, if your upper is a standard AR-15 platform. The FN 36421 barrel is a direct replacement barrel—it uses the same .750-inch gas block diameter and M4 feed ramp design as most modern AR-15 uppers. Confirm that your upper receiver is compatible with a 16-inch carbine-length gas system before ordering.

Is this barrel chrome lined, and what does that do?

Yes, the bore and chamber are chrome lined. Chrome lining protects against corrosion and heat erosion, significantly improving barrel life and maintaining reliable chambering and extraction over thousands of rounds. It's especially valuable if you run suppressed, shoot frequently, or operate in humid or salty environments.

What makes FN's cold hammer forging different?

Cold hammer forging aligns the grain structure of the steel and eliminates internal voids and stresses that can lead to accuracy degradation or premature failure. Every FN barrel is then Magnetic Particle tested to confirm it's free of micro-stresses and defects. This process, refined over more than 100 years, results in barrels that deliver both accuracy and durability.

What caliber and gas system length is this barrel?

The FN 36421 is chambered in 5.56x45mm NATO with a 16-inch barrel and carbine-length gas system.