Barrel Treatments & Coatings
The method of production and the type of steel used to make an AR-15 barrel are just parts of the total equation. Picking the right finishing treatment and/or coating is just as important, lest you suffer a short barrel life and poor accuracy.
Chrome Lining (Interior)
Ah, the classic and the most widely-marketed type of barrel treatment: Chrome lining. This age-old process involves coating the inside of the barrel and chamber with chromium. Chrome lining adds to the physical dimensions of the barrel itself, so the rifling and barrel must be over-sized if this process is to be performed. Once applied, chrome lining is visible as a silvery finish and it measures a few thousands of an inch. It insulates the rifling and barrel, and it's incredibly effective at reducing the effects of rapid-fire and the immense heat it creates.
A properly chrome-lined barrel is easier to clean and it will last longer than a non-coated barrel, usually by a few thousand rounds. With a quality lining, the loss in accuracy usually amounts to just 0.25 MOA, or around 0.25" at 100 meters.
Manganese Phosphate/Parkerized (Exterior)
Chrome lining only protects the inside the barrel, so most lined barrels also get an exterior manganese phosphate treatment, often branded as a Parkerized finish (named after the Parker Rust-Proof Phosphating Company). A chrome-lined-and-phosphate-coated treatment is the most common type of treatment applied to mil-spec barrels and most current, military-issued rifles. 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.
Nitride/Melonite Finish (Interior & Exterior)
Shooters who want the latest hotness should stick with a nitride finish instead. Also called a Melonite finish (which is a brand that performs this type of treatment), the nitride finish effectively coats both the exterior and interior of the barrel. Unlike chromium and manganese phosphate, nitride doesn't "coat" the barrel. Instead, it penetrates the steel structure itself, attaching nitrogen atoms to the atoms in the steel.
This process doesn't add any physical material to the rifling, so the barrel in question doesn't need to be specially fabricated to accommodate this treatment. Nitride is applied by quenching, polishing, and then quenching the barrel a final time in a liquid salt bath. The nitrogen thus penetrates the steel by a few thousands of an inch. A nitride treatment is technically more effective than a chrome-lined finish.
In fact, nitride-treated barrels can withstand direct exposure to water, heat, salt, and corrosive elements better than any other barrel treatment or coating. Nitride is also non-porous and doesn't require any oiling or extra coating to prevent corrosion. A barrel treated with nitride boasts a surface hardness of around 60 HRC, making it the same hardness as hardened tool steel.