AR-15 Bolt Carrier Groups: What Steel & Finish is Best?

Long gone are the days of just buying a generic phosphate-coated, "mil-spec" bolt carrier group. Dozens of combinations of new steels and coatings have hit the market. But some steels and coatings might not be ideal. Others provide genuine improvements in bolt life, reliability, and ease of cleaning.

So, which AR-15 bolt and carrier combo's best? Is the old school phosphate "M16" bolt still the best choice? (Maybe). We're covering it all. Let's get started.

AR-15 Bolt Carrier Steel

This bit's simple: All bolt carriers are made from 8620 alloy, which has been used for decades. 

8620 is a case hardening steel. That means it provides a hardened exterior that withstands scratching, deforming, and pitting, while the core of the part remains softer and more flexible.

The 8620 alloy's ideal for bolt carriers for both these qualities: Its hardened exterior withstands the rotating motion of the bolt and cam pin, and all the repetitive back-and-forth motion the BCG endures when the AR-15 cycles. The softer steel underneath the carrier's surface absorbs the hard impacts of the gas system and buffer assembly without cracking or splitting.

"Are there alternative carrier steels?"

No. Today, all AR-15 (and other AR-type) bolt carriers are all machined from forged 8620 steel. The only differences you'll find between carriers are found in their coatings, which we'll cover below.

AR-15 Bolt Steels

The bolt that comes with your BCG comes in two flavors: Carpenter 158 steel, and 9310 steel. There's quite a bit of misinformation on which steel is superior. We wrote a detailed guide comparing C158 and 9310 (read it here), but the simple summary is this:

There is no meaningful between between C158 and 9310.

BCG Coatings Compared

There are loads of different BCG coatings available, including chrome, phosphate, nitride, nickel boron, Diamond-Like Carbon (also called DLC), titanium nitride (also called TiN), and NP3. Let's compare.

Chrome

The first AR-10 and AR-15s produced by ArmaLite and Eugene Stoner all used chrome bolt carrier groups. Chrome was only abandoned in favor of phosphate coatings because manufacturers (and the military) felt the coating was unnecessary and expensive. Chrome BCGs have made a comeback offering smooth cycling, easy cleaning, excellent heat and wear resistance, and high overall reliability.

Phosphate

Manganese phosphate is the standard coating for mil-spec bolt carrier groups.This porous, matte black finish provides excellent wear and heat resistance, while also retaining oil and lubricants to promote smooth operation with sustained firing. Phosphate has a somewhat "tacky", or gritty finish. To improve smoother cycling, many phosphate carriers have a chrome lined channel that allows the bolt to rotate and move forward and backward more easily.

The phosphate coating's ability to retain lubricants and provide high wear resistance must be weighed against its added difficulty in cleaning. The porous surface of the BCG retains carbon fouling more easily, so these BCGs typically require more effort to clean -- though many shooters stand behind phosphate BCGs, if for no other reason than the fact they've been proven through military use for decades.

Nitride

Nitride is, unlike phosphate and chrome, not an actual coating. Instead, it's a process that converts the surface of the BCG's steel with chemicals to produce a harder, slicker finish. Nitride-treated BCGs are generally regarded as easier to clean than phosphate-coated units, while providing "smooth" cycling similar to chrome BCGs. 

Because no coating is added to the surface of the bolt and carrier, the nitride process also ensures the BCG's machined dimensions remain true to specification. This reduces the risk of tolerance stacking, headspace issues, and tight clearances between components like the firing pin, gas rings, and cam pin.

Today, nitride bolt carrier groups remain the standard choice for many AR-15 part manufacturers and rifle makers, and they're some of the most popular units by annual sales.

Nickel Boron (NiB)

Nickel Boron, or "NiB," looks and feels similar to chrome. Not surprisingly, NiB bolt carrier groups perform similarly to chrome BCGs. They clean easily, cycle smoothly, and provide an ultra-hard, slick, non-porous surface that excels at withstanding heat and wear. NiB carriers are said to function reliability with little to no lubricant (though regular lubrication is still recommended).

NiB does suffer some drawbacks. Over time, the finish is likely to oxidize from carbon fouling and heat, converting its silvery appearance into a tan or brown tone. Some NiB BCGs' finishes have been shown to corrode and separate from the metal below, so regular cleaning is still recommended.

NP3

NP3 is a coating that combines PTFE -- yes, that stuff found on non-stick cookware -- with nickel and phosophorus. This coating produces an incredibly smooth, semi-gloss to satin finish with a similar surface feel to that of chrome, NiB, and nitride. But unlike those other slick coatings, NP3 provides some level of actual "dry lubrication," functioning with high reliability without the need for oil or wet lubrication.

Because of its, NP3 is ideal for guns operating in dusty and sandy environments wherein wet lubrication can attract small particles and potentially cause issues with cycling.

Diamond-Like Carbon (DLC)

If you love GLOCKs, Diamond-Like Carbon, also called "DLC," may be familiar to you. This carbon-based coating provides an incredibly smooth, glossy finish for bolts and carriers, promoting increased lubricity and excellent corrosion and wear resistance. Like NP3, it is also a self-lubricating finish.

DLC makes cleaning your BCG easier, and it helps ensure your rifle runs smoothly, even when fouled. Although technically an additive coating, DLC is applied as an incredibly thin layer, usually measuring just a few microns thick.

Dispelling BCG Coating Myths

There's plenty of internet lore surrounding AR-15 bolt carrier group coatings. Many of them are misleading or simply false. Let's cover some of the popular ones.

Myth: "Nitride bolt carrier groups are fragile because the nitriding process ruins the heat treatment!"

Reality: All BCGs are heated when coated. Nitride bolt carrier groups are favored by most manufacturers. 

Some have claimed that nitride-treating a bolt carrier group negatively affects the steel's heat treatment, reducing the life of the bolt and carrier. Although the steel must be heated to hundreds of degrees to be nitrided, so too must all other BCGs receiving other coatings. 

Chrome, phosphate, DLC, NP3, and nickel boron coatings require the steel to be heated to hundreds of degrees for they can be applied.

If nitriding BCGs caused them to become weak, there would be widespread reports of failures, just as there would be reports of other types of coated BCGs failing due to being heated in the same manner.

As of this article's publication, we have never had a report from any customer that a nitride-treated BCG provided in any of our upper receivers or AR-15 kits has failed.

Myth: "Nitride bolt carrier groups are frequently out of spec!"

Reality: BCGs are fabricated based on what treatment or coating they will receive.

Others have claimed the three bore -- the frontal opening of the carrier wherein the bolt, gas key port, and gas rings reside -- on nitride BCGs are machined too large, because the carriers are machined to accept a heavier coating, instead of a nitride treatment.

But BCGs are machined based on the coating or treatment they will receive. Bolts and carriers that receive heavy chrome or other additive coatings are machined to ensure their dimensions are accurate after they're coated. This applies for nitride-treated BCGs. They're machined to their final dimensions; nitride does not add any material to their surfaces.

Myth: "Nickel boron BCGs have headspace issues because the coating builds up!"

Reality: Nickel boron is an additive coating, just like phosphate, chrome, DLC, and NP3.

Some have claimed that nickel boron causes headspace issues with AR bolts, because the coating adds material to the face of the bolt. This is false in the same manner that some nitride BCGs are machined out of spec. 

Were this true, all other additive coatings -- including DLC, chrome, NP3, and phosphate -- applied to the AR's bolt face would also cause headspace issues.

Myth: "Nickel boron causes gas keys to loosen and corrode!"

Others have claimed NiB-coated carriers suffer corrosion, particularly between the mating surfaces of the gas key and carrier. This only occurs if the carrier isn't cleaned properly. The myth stems from the misconception that NiB BCGs don't need to be cleaned. This is false.

Nickel boron-coated bolt carrier groups must be cleaned regularly, like any other BCG. And, like any other BCG coating, nickel boron can fail if carbon fouling is left to sit on the surfaces of the steel for long periods of time. No coating is totally impervious to fouling and corrosion.

So, Which Coating is Best?

Despite what you might read on gun forums, there is no universally "best" bolt carrier group coating. Each has its pros and cons.

If you want "the most reliable BCG ever," stick with phosphate.

Phosphate-coated BCGs are "mil spec" and, unlike most other AR-15 components, "mil spec" actually means something, here. These units have been proven in service for decades, functioning reliably for tens of thousands of rounds, on average, in rifles like the M16 and M4.

Although phosphate requires more frequent cleaning, its benefits far outweigh this single drawback. This coating retains oil better than any other, ensuring it (and the steel beneath it) remains well insulated from fouling and corrosion. This also ensures the bolt and carrier cycle reliably, even when dirty.

If you want a proven, affordable BCG, nitride is great.

Although not "mil spec," nitride-treated BCGs remain the most popular commercial units on the market today. Every reputable bolt carrier manufacturer produces nitride bolts and carriers for at least one major firearm maker, and many makers produce their own in-house for their retail rifles.

Nitride is incredibly easy to clean and, since it's not an additive coating, its eventual wear won't cause issues with tolerances between the bolt face and chamber, or gas rings and carrier.

If you love mag dumps, go for chrome or nickel boron.

Chrome BCGs can withstand high heat and rapid fire better than any other coatings. This is one reason why Eugene Stoner originally chose chrome as the coating for the AR-15's bolts and carriers (the first rifles made by ArmaLite were select fire, with fully automatic function).

Nickel boron provides similar performance, with high resistance to heat, and a high surface hardness that reduces the chances of the coating failing when subjected to frequent rapid-fire cycling.

If you hate cleaning, choose NP3 or DLC.

NP3 and DLC are the two most corrosion-resistant coatings, and both provide incredible protection against fouling. They're also the only BCG coatings that provide "dry lubricity," meaning they can function reliably without frequent oiling. They're the best coatings if you prefer simply stowing your AR-15 dirty, with little to no cleaning nor maintenance.

(Or combine two coatings)

We've settled on a unique, M16-profile bolt carrier group that combines a phosphate-coated bolt with a nitride-treated carrier (available here). 

We've found this combination provides the best performance and long-term reliability. The phosphate-coated bolt provides great oil retention to ensure it remains lubricated during cycling. The phosphate finish also helps the bolt face and extractor to more effectively grip cartridge casings to ensure good cycling.

The nitride carrier's smoother finish ensures the bolt's cam pin can rotate more freely when fouled, and it also ensures the seals between the carrier key, the bolt's gas rings, and the carrier's bore never fail, while also making cleaning easier.