Understanding Metals, Finishes, and Coatings on the AR

Aluminum alloys are very important in the AR platform. The light weight, machinability, and affordability of aluminum make it the primary metal for larger metal components, like the lower receiver, upper receiver, handguard, and receiver extension.

Aluminum alloys come in different compositions and tempering. The most important of these is 7075 T6. This alloy is used as the primary material for receivers, receiver extensions (except commercial spec; see our What the Spec article for more information), and some lightweight bolt carriers. Any of these components forged or milled from 6061 aluminum should be avoided; 7075 is denser, stronger, stiffer, and harder than 6061, which is needed in these important components.

Less critical components use a variety of alloys, including 6061. This is not a bad thing; as long as it is not one of the components listed above, the alloy used is not as important.

Aluminum can be found in the following AR components:

• Lower Receiver
• Upper Receiver
• Free Float Handguard
• Dust Cover
• Charging Handle
• Receiver Extension
• Lightweight Bolt Carrier
• Lightweight Pivot/Takedown Pins

Alloy steel is one of the most important metals in your AR. It’s hardness, stiffness, and durability makes it the default for many high-pressure, high-impact, and high-friction components.

Steel is an alloy of iron and carbon. Alloy steel is steel alloyed with a variety of other elements, typically comprising between 1% and 50% by weight.

The elements used will influence the physical and chemical properties of the alloy, including machinability, hardness, corrosion resistance, and strength.

The alloy elements used will vary by the designation. The following alloys are commonly used in the AR platform:

• 41xx (e.g. 4130, 4140, 4150): barrel, gas key, gas key screws, extractor
• 43xx (e.g. 4130): cam pin, extractor
• 86xx (e.g. 8620): bolt carrier
• 93xx (e.g. 9310): bolt*
• C158: bolt*

* Be wary of bolts that are not made from 9310 or C158 steel. 4140 is an inferior alloy in this application. The hardness of 9310 steel is between 58 and 62 HRc; the hardness of 4140 is 28 to 32 HRc. The decreased hardness is not a good thing for the high-stress and high-pressure environment of the bolt. And if the difference in physical attributes are not convincing enough, a 4140 bolt DOES NOT MEET MIL-SPEC STANDARDS.

Alloy steel can be found in the following AR components:

• Bolt Carrier
• Bolt
• Barrel
• Muzzle Device
• Crush Washer/Timing Shims
• Gas Block
• Gas Tube
• Dust Cover
• Forward Assist
• Pivot/Takedown Pins
• Magazine Catch
• Bolt Catch
• Trigger Group
• Safety Selector
• End Plate
• Castle Nut
• Screws/Bolts
• Springs
• Detents

Stainless steel is a metal that is designed for corrosion resistance. It has a higher percentage of chromium than other alloy steel, which forms a chromium oxide layer on the outer surface, which inhibits corrosion and rust.

While stainless steel can be used in place of other alloy steel in many springs, pins, and bolts, it’s most conspicuous use is in barrels. There are generally two types of stainless steel used in barrels. 416 and 416R. 416R is superior to 416 due to reduced sulfur and increased molybdenum content. This difference in composition makes 416R harder than 416, and thus, more resistant to corrosion and wear.

Stainless steel can be found in the following AR components:

• Barrel
• Muzzle Device
• Gas Block
• Gas Tube
• Firing Pin
• Pivot/Takedown Pins
• Screws/Bolts
• Springs
• Detents

Titanium is known as one of the strongest metals per unit of weight. It’s alloys are lighter than steel and is harder and stronger than both steel and aluminum alloys. That said, this metal is also less rigid and more brittle than steel, so it must be used thoughtfully. Used appropriately in place of steel and aluminum, titanium alloy can shed weight from an AR build.

Titanium can be found in the following AR components:

• Lightweight Bolt Carrier
• Lightweight Pivot/Takedown Pins
• Screws/Bolts

Anodizing is an electro-chemical oxidation process that results in an unnaturally thick oxidation layer on the surface of a metal, most commonly aluminum. Anodizing is typically performed to improve the resistance to wear and corrosion by increasing the surface hardness of the metal.

Anodizing is performed by running an electric current through an electrolytic acid solution and through the object itself. Metal ions (for aluminum, this is Al₃⁺) pass from the object into the electrolyte solution (the acid), while superoxide (O₂^–) ions simultaneously diffuse into the metal. The resulting thick oxide layer is significantly harder and more stable than the unoxidized metal and the natural oxide layer.

Type III anodizing, or hardcoat anodizing, is based on the military standard Mil-A-8625. This level of anodization results in an oxide layer that is up to 75,000 times thicker than the natural oxide layer. Type III anodizing is very important to structural components of an AR, as well as any components that will experience significant amounts of stress or abrasion. Type III anodizing should be used as the minimum acceptable surface finish for the following aluminum components:

• Lower Receiver
• Upper Receiver
• Handguard (except plastic drop in)
• Receiver Extension
• Lightweight Aluminum Bolt Carrier Groups

Blackening of firearm components is typically either done by a hot process (hot black oxide) or a cold process (cold bluing).

Hot Black Oxide

Black oxide, or blackening, is a chemical conversion of iron into a black oxide called magnetite (Fe₃O₄). While a type of iron oxide, magnetite is not rust (Fe₂O₃), a red oxide. By proactively oxidizing the iron to black oxide, you can preempt the formation of red oxide.

In this process hot baths of sodium hydroxide, nitrates, and/or nitrites are used to convert the surface steel to magnetite.

The resulting finish imparts mild corrosion resistance and decreases the shine of the metal. It can be found as a finish on barrels (though uncommon in the AR platform) and crush washers.

Cold Black Oxide (a.k.a. cold bluing)

Cold black oxide is not actually an oxide conversion finish, like hot black oxide. This finish consists of copper selenide deposited on the surface of the steel. This finish imparts very little change to the physical properties of the steel. This finish is rarely seen on the AR platform.

Known as phosphate, manganese (or zinc, depending on the chemical process) phosphate, phosphatized, or Parkerized, this is the Mil-Spec (and therefore, the default) coating for many steel components in the AR platform.

The phosphate coating is a chemical conversion coating that is the result of soaking a steel part in a bath of phosphoric acid with soluble zinc (zinc phosphate) or manganese (manganese phosphate) salts. The resulting finish is a hard, matte black coating precipitated onto the surface of the steel. The finish also improves the acceptance of lubricants vs. uncoated steel, which is useful for the moving parts of the AR platform.

Phosphate coated steel can be found in many components of the AR platform, including:

• Muzzle Device
• Crush Washer
• Barrel
• Gas Block
• Bolt Carrier Group
• Dust Cover
• Forward Assist
• Pivot/Takedown Pins
• Magazine Catch
• Bolt Catch
• Bolt Catch Buffer
• Trigger, Hammer, and Disconnector
• Hammer/Trigger Pins
• Trigger Guard
• Safety Selector Switch
• Buffer Retainer Pin
• Receiver Extension End Plate
• Receiver Extension Castle Nut

Known as nitride, ferritic nitrocarburizing, Melonite, Tenifer, Tifftride, QPQ (quench-polish-quench), and salt bath nitride, this is a common upgraded finish for steel components on the AR platform.

Nitride finish is the result of infusing nitrogen into the surface of a metal, creating a case-hardened surface. The nitriding surface involves dipping steel into a bath of hot nitrogen salts. The resulting finish is very hard, and has good resistance to corrosion and wear.

Many steel parts in the AR platform can be nitrided for improved resistance to wear and corrosion, including:

• Muzzle Device
• Barrel
• Gas Block
• Gas Tube
• Bolt Carrier Group
• Pivot/Takedown Pins
• Magazine Catch
• Bolt Catch
• Trigger, Hammer, Disconnector
• Hammer/Trigger Pins
• Safety Selector Switch
• Buffer Retainer Pin

Chrome plating is an electro- or chemically-plated finish applied to the exterior of a metal.

Chrome plating is typically applied by electroplating chromium onto the surface of a metal.

A chrome finish is very hard, and is resistant to heat, corrosion, and wear. It also imparts reduced friction. Because of these characteristics, it is the Mil-Spec finish for certain component surfaces, like barrel bore, bolt carrier lining, and gas key lining.

Chrome plating may be found on the following AR components:

• Barrel (hard chrome lining)
• Gas key (hard chrome lining)
• Bolt Carrier (hard chrome lining)
• Bolt Carrier Group (hard chrome exterior)
• Various Small Parts (cosmetic chrome)

Nickel boron, also known as NiB and electroless nickel-boron plating, is a metal plating of a nickel-boron alloy applied to a substrate.

Unlike chrome plating, NiB coating does not require electrical current. The application of NiB involves dipping an object into a bath containing nickel salt and a boron-containing reducing agent (e.g. sodium borohydride).

NiB is significantly harder than unfinished steel, on the order of (or in excess of) hard chrome. NiB coating results in a columnar structure of nickel and boron that results in microscopic nodules on the surface of the object. These nodules reduce the surface-to-surface contact between two mating surfaces and, thus, decrease friction, improve heat dissipation, and increase lubricant-carrying capacity. These characteristics make nickel boron a good candidate for bolt carrier finish, which is where you will typically find it on an AR.

Titanium nitride, also known as TiN and Tinite, is an extremely hard ceramic material that can be applied to various metals.

TiN is typically applied via physical vapor deposition. This method involves evaporation/sublimation of a solid (in this case, the TiN) in a vacuum and subsequent condensation/deposition as a thin film on a substrate.

TiN is extremely hard and has a low coefficient of friction. These characteristics make TiN a good coating for bolt carrier groups.

Visually, TiN is typically anywhere between a lustrous gold finish to a satin orange-yellow finish. If you don’t mind a little flash, this can look pretty cool peaking out of the ejection port.

Diamond-Like Carbon, or DLC, is a state of the art finish that, as the name implied, is extraordinarily hard.

As with TiN, DLC is applied via physical vapor deposition. This method involves evaporation/sublimation of a solid (the sp³ hybridized carbon atoms) in a vacuum and condensation/deposition as a thin film on a substrate.

DLC is the hardest finish as you will find on the AR platform and it is extremely durable. It has superior lubricity compared to every other finish (even running dry). If you have the money to spend, you can’t go wrong with a DLC-coated bolt carrier.

Hydro dipping, also known as water transfer printing, allows the application of printed designs to three dimensional surfaces. This finish can be used to apply patterns and graphics to AR components.

This finish typically begins by first printing a pattern on a hydrographic film. The film dissolves in water, so, when it is placed on the surface of a water bath, the film dissolves, leaving the printed image floating on the surface of the water. When an object is pushed through this layer, the ink adheres to the surface of the object. The applied graphics require a clear coat for protection.

The technique may also involve spraying paint on the surface of a water bath, and otherwise following the process for printed patterns. The applied paint requires a clear coat for protection.

This finish is purely cosmetic and because the patterns are printed, the effects can be pretty sweet. However, this finish imparts zero structural value. It is prone to scratching, chipping, etc., which can make your gun look like sh*t pretty quickly if you don’t protect it.

We don’t need to explain this one. Spray paint is an easy way to change the look of your gun. It can be applied free hand or in patterns using stencils or foliage to impart shapes and designs.

Spray painted parts should have clear coat applied to protect the finish.

As with hydro dipping, this is purely cosmetic and is susceptible to wear, scratching, etc. without the appropriate protective top coat.

DuraCoat is like spray paint on steroids. The proprietary blend formulation offers a robust finish that can easily be applied by the average Joe and does not require specialized equipment, significant surface preparation, baking, etc.

DuraCoat is offered in over 300 colors, with various additives for texture and appearance. It is offered as a liquid for use with spraying equipment, or in aerosol cans for the DIY-er.

Because DuraCoat is offered in convenient spray cans, it is as easy to apply as normal spray paint. Unlike hydro dipping and spray painting, DuraCoat finishes do not require any clear coat and the finish is much more resilient. So, if you are thinking about hydro dipping or spray painting your gun, we strongly recommend that you consider using DuraCoat instead; for a comparable level of effort, you will get longer-lasting results.

While leaps and bounds better than hydro dip and spray paint, DuraCoat is less durable than baked on finishes, like KG Gun Kote and Cerakote.

KG Gun Kote is a baked on finish that is relatively easy to apply to any substrate.

KG Gun Kote is offered in nearly 100 colors/finishes. It is offered as a liquid for using with spraying equipment.

Given the need for application equipment and baking equipment, you could consider KG Gun Kote to be a bit less DIY-friendly than DuraCoat, but more DIY-friendly than Carakote.

KG Gun Kote is generally more robust than DuraCoat, but cannot compete with attributes of Cerakote.

Cerakote is a baked on ceramic-polymer coating that can be applied to metals and plastic.

The physical characteristics of Cerakote are superior to all other firearm coatings. It is harder, more corrosion resistant, abrasion resistant, and chemical resistant, and has the best rating for adhesion.

Cerakote is offered in hundreds of colors and comes in a slurry/suspension.

For all of the benefits of Cerakote, it is NOT DIY-friendly. Unless you are adventurous and don’t mind effing up hundreds of dollars worth of components, you will probably need to find a competent/authorized applicator who has the knowledge/experience, as well as the equipment to apply this coating properly.