Nitride vs. Chrome Lining: Which AR-15 Barrel Finish is Best?

Whether you build or buy a rifle, picking out a barrel is arguably the most important thing you'll do. After all, the barrel determines how your rifle performs; everything else comes second. In addition to choosing between carbon and stainless steel, and your barrel's type of rifling, you've got to pick what finish -- if any -- your barrel comes with.

When it comes to the bore, there are two barrel treatments to pick from: Chrome lining, and nitride (sometimes called Melonite or Tennifer). Phosphate (also called Parkerizing) is an exterior coating that can substitute nitride, which is applied to both the bore and exterior of the barrel.

Each provides certain advantages, and potential disadvantages, when it comes to increasing the life of your barrel and enhancing its accuracy and reliability.

Chrome Lining

This is the most durable barrel coating. It provides the most protection and life expectancy.

Chrome linings were first applied to rifle barrels by the Japanese during WWII; the Type 99 Arisaka and Type 11 submachine gun were the first weapons to be fielded with them. The process was later refined by Colt and became a component of mass U.S. weapon production in the early 1970s. Originally designed to improve the M16's reliability when it came to extracting casings and protecting against corrosion, chrome lining was found to greatly enhance a barrel's ability to withstand heat from rapid and automatic fire.

Chrome lining is an additive coating applied through electrolysis, meaning it adds dimension to the bore. For a barrel to receive a chrome lining, its bore and rifling must be undersized so that when the coating is applied, the bore is the correct diameter for the caliber in question.

To coat a barrel with chrome, it must first be electrically charged to create a cathode. It's then submerged into an electrolyte bath that contains chromium particles. The positively charged barrel causes chromium in the liquid solution to bond to the bore and solidify, producing a matte silver coating.

Chrome Lining Advantages

Chrome lining is the toughest coating you'll find on any gun barrel. It produces an incredibly hard, insulating layer that protects the bore and its rifling from throat erosion and degradation due to heat. It's the coating of choice for military-issued rifles and machineguns, and will greatly extend a barrel's life by thousands of rounds.

A chrome-lined barrel is also easier to clean. Carbon fouling and copper deposits don't stick well to this slick, non-porous coating. Because it physically separates the bore's steel from these contaminants, chrome all but eliminates the risk of corrosion from spent gunpowder damaging the bore.

Chrome Lining Disadvantages

Because it is deposited directly onto the bore's rifling, and because the rifling must be undersized to accept the coating, chrome lining can reduce accuracy by unevenly filling in the rifling's lands and grooves. A quality lining will typically result in only a minor loss in accuracy -- usually 0.25 to 0.5 MOA when compared to a non-chromed barrel -- but this coating should not be considered when the goal is to maximize long-range accuracy with a slow firing schedule.

Chrome lining is also more expensive to apply than nitride, which adds to the final cost of the finished barrel. So, if you're looking for better accuracy at a lower price, chrome bores should be avoided.

Nitride

Also called Melonite and Tennifer, nitrocarburizing, or simply "nitriding," treats both the exterior of the barrel and the bore inside. Unlike chrome, nitride isn't an additive coating. Instead, nitriding is a chemical treatment that penetrates the surface of the steel, attaching nitrogen atoms and additional carbon atoms to the metal's existing carbon and iron matrix.

Nitride is applied by by immersing the barrel in a molten salt bath containing nitrogen and carbon compounds. The intense heat of the bath makes the barrel's raw steel reactive, allowing the carbon and nitrogen atoms to bond to it. After submersion, the barrel is quenched and rapidly cooled, locking the newly added atoms into the steel's atomic structure.

Nitride Treatment Advantages

Nitriding doesn't add any dimensional material to the bore, so the barrel's rifling doesn't need to be undersized to accommodate the treatment. This also ensures the rifling's shape and dimensions remain as accurate as they were when machined, promoting greater accuracy. A nitrided barrel obtains a surface hardness of around 60 HRC, making it the same hardness as tool steel. Like chrome, this increased surface hardness reduces throat erosion and rifling wear.

Nitrided barrels are also significantly more resistant to corrosion from carbon fouling and spent gunpowder than untreated barrels. And, like chrome, nitride reduces the steel's porosity and adds lubricity. Carbon and copper deposits are far less likely to stick to the bore, making it easier to clean.

Nitride Treatment Disadvantages

Although it extends barrel life and promotes heat resistance, nitride-treated bores are still more susceptible to throat erosion and wear than chrome-lined bores. It's simply a less durable treatment, so it won't increase barrel life as significantly as chrome.

So, Which Is Better? Chrome or Nitride?

Neither chrome lining nor nitriding is universally best. Chrome-lined barrels will last longer, at the expense of some accuracy. Nitride barrels won't last as long, but they'll still enjoy a long barrel life and they'll provide better accuracy.

If your goal is to own a "SHTF" gun with a barrel that can handle repeated magazine dumps and still outlast you, go for chrome lining. If you want a barrel that will still last for thousands of rounds and provide better accuracy downrange -- and if you want to save some money for other parts, upgrades, optics, or ammo -- stick with nitriding.

What About Phosphate?

Chrome lining only protects the inside of a barrel, and barrels with chromed bores can't be nitrided. So, chrome-lined barrels get an exterior coating made of manganese phosphate. This is a matte, porous finish that only ever coats the outside of the barrel. Many affordable barrels with untreated bores also receive phosphate coatings to protect their exteriors against moisture and corrosion.

Although it's a basic coating, it remains the mil-spec treatment for military rifles like the M4 and M16. Some like the phosphate coating because its matte, porous structure aborbs and retains grease and oil, which forms a natural barrier against oxidation.

Phosphate Coating Advantages

A phosphate finish is incredibly resistant to heat and the elements, but it is also slightly porous. It requires occasional oiling to effectively prevent corrosion.

Phosphate Coating Disadvantages

Phosphate isn't applied to the bore of a barrel, so it provides no protection for the rifling inside. Its porous structure also means it requires occasional oiling. Otherwise, it will instead absorb and retain moisture, eventually resulting in oxization forming on the surface of the barrel.