Which AR-15 BCG Finish is Best?

The classic phosphate-coated BCG reigned supreme in the land of AR-15s for decades. But that's not the case anymore. Now, there are half a dozen different coatings and finishes to choose from.

Bolt Carrier Group Finishes Compared

Some of our customers ask, "Which BCG coating's the best, overall?" Let's review.

Phosphate / Parkerizing

Type: Conversion coating

The Manganese Phosphate coating is the standard coating found on service-issued bolt carrier groups. In other words, this is the coating you want if you're investing solely in "mil-spec" parts.

Pros: Phosphate is a conversion coating, so it doesn't add dimensions to the finished bolt carrier groups. This ensures the machined tolerances of the bolt and its carrier remain in spec. Because it creates a porous, rough surface, Phosphate retains oil and lubricants better than any other BCG coating. 

That means lubrication doesn't need to be applied as frequently. Being a conversion coating, phosphate cannot chip or flake off the surface of the steel. For these reasons, Phosphate-coated BCGs are considered some of the most reliable units when it comes to running your AR "dirty" for long periods of time.

Cons: The only downside of investing in a phosphate-coated BCG is the maintenance: Just as it retains oil, phosphate tends to retain fouling from the AR's gas system. Deeper cleaning is occasionally required to remove this fouling.

Nitride

Type: Case hardening treatment

Like Manganese Phosphate, the QPQ Nitride treatment is a type of conversion coating. The process case hardens the surface of the BCG's raw steel, significantly improving its wear resistance and promoting natural lubricity. 

Pros: Nitrided BCGs are very slick and provide smooth, reliable cycling, even when fouled. And, like Phosphate, the Nitriding process doesn't add any dimensional thickness to any components, ensuring the original specifications of the bolt and its carrier are maintained. This coating suffers no risk of chipping or flaking, either.

Nitride is incredibly easy to clean, too. Fouling doesn't cling to this treated surface, so the bolt and carrier can be cleaned relatively quickly, without deep scrubbing.

Cons: There are no meaningful drawbacks to consider with a nitride BCG. Although some "experts" on the internet have made claims that nitride-treated BCGs suffer from brittle steel due to the treatment process, these claims are unfounded. 

In fact, nitride BCGs are now the most popular units on the commercial market. Virtually all major manufacturers use nitride BCGs in their AR-15s.

Chrome

Type: Additive coating

Chrome is the first ever coating used on the AR-15's bolt carrier group. Chrome was only abandoned in favor of phosphate due to manufacturing costs.

Pros: Chrome was favored as a BCG coating by Eugene Stoner -- the inventor of the M16 and AR-15 -- for its superior heat resistance, incredibly high wear resistance, and high natural lubricity. It also resists fouling, can be ran "dirty" for extended periods of time and, when maintenance needs arise, it cleans about as easily as nitride.

Cons: Chrome is an additive coating, so it adds dimensional thickness to both the bolt and carrier. This must be compensated for by the manufacturer by undersizing the free bore of the carrier -- that's the space wherein the bolt rests to form a gas seal -- and the bolt face.

Failure to do so will result in a "sticky" bolt, and potential headspace issues. Chrome can also chip, crack, and wear over time, though this isn't expected to happen for thousands of rounds, so long as occasional lubrication and basic maintenance are applied.

Nickel Boron (NiB)

Type: Additive coating

Nickel boron-coated BCGs could be considered a modern equivalent to the original chrome BCG. This additive coating comprises 95% nickel and 5% boron, creating a surface with high hardness, high lubricity, and high heat resistance.

Pros: Nickel boron performs like chrome. It's slick, it promotes smooth cycling, it's easy to clean, and it withstands the high heat generated by rapid and automatic fire.

Cons: Although nickel boron maintains a hard surface, it can suffer from carbon tarnish. This is a process wherein carbon fouling forms deposits in the surface of the coating, discoloring it. This does not affect the bolt's reliability in any way, though some users might dislike their BCG appearing discolored over time. Because nickel boron is so slick, it requires more frequent lubrication, too.

Lastly, some gunsmiths have claimed nickel boron is an unreliable coating. But, like any other additive coating, this unreliability is only caused by bolts and carriers being machined without accommodating for the coating's added dimensional thickness. 

So long as the bolt and carrier being coated are machined properly before coating, there is nothing specific about nickel boron that will cause issues with reliability. 

Because of these machining tolerance issues, nickel boron has earned a bad reputation; many manufactures no longer use NiB BCGs for this reason.

Diamond-Like Carbon (DLC)

Type: Additive coating

Diamond-Like Carbon is a lesser known coating, but it's been in use for decades. DLC is a nano-particle coating made of carbon that forms a crystalline, diamond-like structure on the steel's surface. This coating is applied by chemical vapor deposition, creating one of the thinnest additive coatings on the market. It measures just a few microns thick.

Pros: DLC is touted as one of the hardest BCG coatings available. It also provides some of the highest natural lubricity of any coating, requiring very little lubrication. It boasts an incredibly low drag coefficient, ensuring the bolt and carrier run smoothly. And, unlike nickel boron, DLC doesn't shed its lubrication as readily. But when maintenance is required, it is said to be the easiest to clean of any BCG coating. Lastly, because DLC is applied as such a thin layer, there are no concerns with added dimensional thickness, nor part tolerances being made out of spec.

Cons: The only drawback of a DLC-coated BCG is cost. This is a "niche" coating, and just a few BCG manufactures offer it. The cost of application is higher than other coatings, so the resulting bolt and carrier will typically demand a premium. But, if cost is no object, a DLC bolt carrier group is arguably one of the best options on the market today.

NP3 (Teflon & Nickel)

Type: Additive coating

NP3 is not unlike nickel boron. It's an additive coating comprising between 80% and 84% nickel. But instead of using boron, it comprises 9% to 11% phosphorous and 7% to 9% polytetrafluoroethylene, known by most as Teflon. 

Pros: NP3 is regarded by most users as one of the best coatings for BCGs. It performs similarly to DLC, albeit with slightly lower surface hardness. Its use of Teflon ensures NP3 provides incredibly high natural lubricity and easy cleaning. It also provides high heat resistance.

Cons: Like other additive coatings, NP3 requires both the bolt and carrier be machined to compensate for the coating's added dimensional thickness. The coating can also wear over time -- usually only after many thousands of rounds -- so occasional lubrication and cleaning are still required. Few manufacturers are producing NP3 BCGs, so these units tend to be most costly, too.

Titanium Nitride (TiN)

Type: Additive coating

Titanium Nitride is recognizable by its gold color. Although comprising titanium and nitrogen, TiN is technically a ceramic. It's an incredibly hard-wearing surface treatment applied through positive vapor deposition, producing a coating just a few microns thick.

Pros: TiN BCGs clean incredibly easily. Their ease of maintenance can be compared to that of NP3 and DLC. Because the coating is so thin, there are no dimensional concerns to consider with headspacing and the seal between the bolt's gas rings and the carrier's bore.

Cons: Some simply despise the gold color of TiN BCGs. When clean, the coating's appearance is bright, and typically somewhat reflective. TiN BCGs have also earned a poor reputation, not unlike nickel boron units -- though this is largely due to low-cost manufacturers not affording good quality control. The coating itself is inherently reliable.

So, Which BCG Coating is Best?

Not surprisingly, the phosphate or nitride BCG is likely the best choice when it comes to cost, performance, and proven reliability. Both finishes have been in wide use for decades, and these are the BCG coatings we offer our customers in all our of our AR-15 kits.

Of the tens of millions of AR-15s currently in circulation in the United States, most rifles are using one of these two BCGs. Because phosphate and nitride are both conversion coatings, neither will measurably wear over the life of the bolt carrier group. 

For those who prefer easy maintenance, infrequent lubrication, and smooth cycling, no matter the cost, DLC- and NP3-coated BCGs are favored. Units with these coatings enjoy the best overall lubricity and resistance to fouling, and manufactures that produce these units tend to adhere to strict machining tolerances with extensive quality control.