FN 20-100043 AR-155.56x45mm NATO 20" Button Rifled M16 Profile Rifle Length Gas System, Black Phosphate Cold Hammer Forged Chrome Lined

The FN AR-15 5.56x45mm NATO 20" barrel with M16 profile and rifle-length gas system is a purpose-built upper receiver component for shooters who demand mil-spec reliability and accuracy. Built on the proven FN platform used by military and law enforcement worldwide, this barrel handles everything from precision rifle work to high-round-count training sessions. The 20-inch length and rifle-gas configuration deliver the velocity and dwell time that make 5.56 NATO sing, whether you're running a dedicated match gun, a classic A4-style build, or a versatile duty rifle.

FN manufactures this barrel from U.S.-sourced Chrome Moly Vanadium steel to Mil-B-11595E specification, then subjects every unit to High Pressure Testing (HPT) and Magnetic Particle Inspection (MPI) to eliminate hidden stress risers and material flaws before it ships. That quality control matters: it's the difference between a barrel that shoots groups and one that shoots reliably for thousands of rounds. The bore and chamber are chrome-lined to fight corrosion and heat erosion, extending barrel life and maintaining consistent headspace and feeding over a long service life. M4-style feed ramps and barrel extension ensure reliable chambering and extraction across commercial and mil-spec ammunition.

The black phosphate finish is tough—it resists external corrosion and oxidation without the weight or maintenance burden of other protective coatings. This is a no-nonsense, duty-grade barrel that works with any mil-spec AR upper receiver and lower, and pairs seamlessly with standard carbine or rifle-length handguards. If you're building or upgrading an AR for accuracy, reliability, or both, this FN barrel is a smart investment in a rifle that will last.

Key Features

• 20-inch M16 profile with rifle-length gas system for optimal velocity and dwell time
• Cold hammer-forged from U.S. Chrome Moly Vanadium steel per Mil-B-11595E
• High Pressure Tested (HPT) and Magnetic Particle Inspected (MPI) for consistency and durability
• Chrome-lined bore and chamber resists corrosion and heat erosion, extends barrel life
• M4-style feed ramps for reliable feeding and extraction
• Black phosphate finish for external corrosion resistance
• Mil-spec compatible with standard AR-15 uppers and lowers

Specifications

Frequently Asked Questions

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

Yes. This is a mil-spec barrel with a standard barrel extension and M4-style feed ramps, so it's compatible with any mil-spec AR-15 upper receiver and lower. It threads into your receiver with a standard barrel nut.

What's the difference between rifle-length gas and carbine-length gas?

Rifle-length gas systems have the gas port located further back on the barrel, which creates a longer dwell time (the delay between bullet passing the port and the round being extracted). This typically results in softer recoil impulse, lower bolt carrier velocity, and reduced wear on components—a good choice for accuracy work or high-round-count shooting.

Why is this barrel chrome-lined, and does that affect accuracy?

Chrome lining protects the bore and chamber from corrosion and heat erosion, which significantly extends barrel life and keeps your rifle reliable through thousands of rounds and various storage conditions. The impact on accuracy is negligible for most shooters; FN's quality control and manufacturing process ensure that function and durability don't come at the cost of performance.

What testing does FN perform on this barrel?

Every barrel undergoes High Pressure Testing (HPT) to verify it can safely handle proof loads, and Magnetic Particle Inspection (MPI) to detect internal flaws and stress risers in the material. These tests ensure consistency and reliability from every barrel that ships.