FN 20-100039 AR-155.56x45mm NATO 16" Button Rifled M16 Profile Carbine Length Gas System, Black Phosphate Cold Hammer Forged Chrome Lined

The FN 20-100039 is a cold hammer-forged AR-15 barrel built to military specifications for shooters who demand durability and reliability. This 16-inch 5.56x45mm NATO carbine barrel features an M16 profile and chrome-lined bore—a pairing that extends barrel life, resists corrosion, and stands up to sustained firing and harsh conditions. FN barrels start with U.S.-sourced Chrome Moly Vanadium steel and subject every unit to High Pressure Testing (HPT) and Magnetic Particle Inspection (MPI) before shipping, eliminating micro-stresses and flaws that compromise accuracy or safety.

Whether you're building a patrol rifle, upgrading an existing AR, or assembling a reliable 5.56 platform for duty or sport, this barrel delivers the fit and function that comes from FN's military-grade manufacturing. The black phosphate finish guards against external rust and oxidation, while the M4-style feed ramps and barrel extension ensure smooth, consistent feeding across quality ammunition. Cold hammer forging creates a denser, more uniform grain structure than button rifling alone—translating to better shot-to-shot consistency and longer service life before throat erosion becomes a concern.

This is a components-grade barrel for builders and armorsmiths, not a complete rifle. You'll need a gas tube, bolt carrier group, handguard, stock, and lower receiver to complete your build. The 16-inch carbine length gas system strikes a practical balance between velocity and controllability in 5.56, making it a sensible choice for modern sporting rifle matches, home defense builds, or mil-spec clones.

Key Features

• Cold hammer-forged Chrome Moly Vanadium steel (Mil-B-11595E spec)
• 16-inch carbine-length barrel for 5.56x45mm NATO
• Chrome-lined bore and chamber for extended barrel life and corrosion resistance
• M4-style feed ramps for reliable feeding
• Black phosphate finish resists external corrosion and oxidation
• High Pressure Tested (HPT) and Magnetic Particle Inspected (MPI) at the factory
• M16 profile with standard AR-15 dimensions for universal fit

Specifications

Frequently Asked Questions

Will this FN barrel fit my AR-15 lower receiver?

Yes. The FN 20-100039 uses standard AR-15 barrel dimensions and threading. It will fit any AR-15 lower receiver that accepts a carbine-length 5.56x45mm barrel.

What gas tube length do I need for this 16-inch barrel?

This is a carbine-length barrel, so you'll need a carbine-length gas tube (also called intermediate) for your build. Do not use a rifle-length or pistol-length tube with this barrel.

Is this a complete rifle or just a barrel?

This is a barrel component only. To assemble a functional AR-15, you'll also need a lower receiver, bolt carrier group, gas tube, handguard, charging handle, stock, trigger, and magazine.

Why is this barrel chrome lined?

Chrome lining protects the bore and chamber from corrosion and heat erosion, which extends barrel life significantly. This is especially valuable if you shoot in humid conditions, use corrosive ammunition, or plan sustained fire.