If you are in the market for new AR-15 parts, specifically a bolt carrier group, you might be a little overwhelmed with the various treatments/coatings available today. A few years ago there were only two options, phosphate or a hard chrome. Now there are over a half dozen options to choose from on the market.
Hard Chrome
Some of the early M16 rifles adopted by the US military had a hard-chromed bolt carrier. Hard chrome is a surface plating that covers the outside of the carrier. These bolt carriers had a natural lubricity and proved to be durable, the only downside being that the hard chrome plating was prone to flaking. This made it not desirable and along with a higher cost, caused the switch to a phosphated coated bolt carrier group.
Phosphate
This is by far the most popular and well-known finish to have on a bolt carrier group. It is a chemical process that is achieved by soaking the parts in a boiling manganese solution, this practice is also known as parkerizing. Phosphate is not a surface coating, but a metal treatment that penetrates the metal which reduces friction and prevents corrosion. It it the current mil-spec to have a phosphated bolt carrier group with the inside of the gas key and carrier where the bolt rides hard chromed. Phosphating is more difficult to get completely clean like nickel boron or black nitride due to the caked on powder and carbon fowling adhering to the finish.
Black Nitride
Black Nitride is becoming more common and slowly replacing the parkerizing of steel parts. Most chrome moly barrels that were parkerized in days past are now melonited, which is a fancy name for black nitride. Nitriding vastly increases the hardness of the metal that is treated without changing the material dimensionally like the build up of a hard coat. This treatment is great for use on barrels to prevent corrosion and increase barrel life. A barrel that is nitrided is going to be more accurate that a chrome lined barrel because chrome plating takes away some of the sharpness of the rifling. The black nitride treatment is very durable and highly resistant to wear and it will not flake because it is not a coating.
Nickel Boron
When searching for a bolt carrier group to purchase, you will run across NiB or nickel boron. Nickel Boron is a surface coating that is increasingly becoming a favorite among individuals wanting an easier to clean bolt carrier group that has a natural lubricity. NiB coated bolt carrier groups are in the middle range on price, more costly than black nitride or phosphate but far less than Titanium Nitride. Even though it is a popular choice, nickel boron can and does flake off from time to time so it is recommended to purchase a high end bolt carrier group from a manufacturer that warranties the coating. Another downside of the nickel boron coated bolt carrier groups is the finish will dull after time and take on a darker carbon color, even if the carrier is clean. The dulling of the bolt carrier does not affect the slickness at all but it does negate purchasing the NiB bolt carrier group for its shiny finish.
Black Nickel
Black nickel is a derivative of nickel boron. When applying, the material is first coated with nickel boron, then it is treated in a salt nitride bath. This process isn’t commonly used due to the expense in treating the bolt carrier twice. This coating is notable because it has all the slickness and ease of cleaning of a nickel boron carrier but the hardness properties of the black nitride making it less prone to flaking.
Titanium Nitride
One of the most expensive coatings to have on a bolt carrier group is titanium nitride or TiN. TiN is a thinly-applied surface coating has a shiny gold-like appearance and is often used for show builds due to aesthetics. Titanium Nitride is often used to coat drill bits or Although it does have a natural lubricity and it cleans easily, this coating offers nothing more than other offerings other than the shiny gold color.
Choosing a bolt carrier group can get pretty touchy because everybody has an opinion of which is the better coating or treatment. All of the bolt carrier coatings or treatments have their trade offs so it is up to the buyer to make the decision on what they want and need. My personal favorite is the black nitride coated bolt carrier groups. It has all the pros like ease of cleaning and wear-resistance without the downside of potential flaking. Even though I own a couple nickel boron bolt carrier groups, I don’t need a shiny parts to look cool. I need cost effective bolt carriers that are strong, protected and don’t wear fast. That’s why I and many others choose black nitride.