Looking for a Bolt Carrier Group (BCG) for your AR 15 build? It can get a bit confusing with so many choices and variations of material and coating available. We’ll make it easy for you by breaking down the leading types of BCG’s here, giving you an idea of what to look for.

AR 15 Bolt Carrier Group Components

M16 Full-Auto Bolt Carrier Groups

You’ll find that there are a wide choice of quality M16 full auto bolt carrier groups on the market. Full auto BCG’s have grown hugely in popularity over the years, and could probably be considered the standard carrier these days. While this is a full-auto part, it is impeccably legal to own and use in your build.

AR-15 Semi-auto BCG’s also exist, however they have less material on the back of the carrier. The extra material (more specifically, an additional lug) on M16 BCG’s allows them to be well-suited with a full auto sear. M16 BCG’s weigh a bit more than Semi-Auto BCG’s, however this extra weight may help your rifle cycle more smoothly.

Mil-Spec Bolt Carrier Groups

A Mil-spec BCG generally translate into the fact that it was built to meet military specifications. For instance, the ODIN Works AR-15 Black Nitride Bolt Carrier Group 5.56/.223/.300BLK peruses true Mil-Spec dimensions and fits AR-15/M4 compatible style Rifles (i.e.) .223/5.56mm, .300 Blackout.

The ODIN Works 6.5 Grendel Type 2 Black Nitride Bolt Carrier Group is also seen as a true Mil-Spec bolt carrier group and is manufactured for Type 2 6.5 Grendel rifles.


Steel Bolt Carrier Groups

Steel is the most common material used in bolt carrier groups because it is durable, heat resistant, and is able to withstand wear and tear well. As an added plus point, it’s not too expensive.

Different types of steel are used for different BCG parts. For instance, you can get a Billet carrier machined from 8620 steel, a gas key made from 4130 steel, and a firing pin manufactured from 8740 steel in a single bolt carrier group, as is the case with Aero Precision’s .308/7.62 bolt carrier group. The right steel grade for one component may not be best for another. For instance, carriers are often made from 8620 steel, but this type is too weak to be used for a bolt.

Steel BCG’s are flawlessly suitable for many AR-15 enthusiasts, and it’s also commonly used by military and law enforcement personnel.

Aluminum Bolt Carrier Groups

Aluminum is a lighter material that is often used for low mass bolt carrier groups. It is fairly strong but may not last as long as steel does. Aluminum BCG’s tend to be popular for shooting competitions and often work well with guns that have adjustable gas systems.

Titanium Bolt Carrier Groups

Titanium BCG’s are lighter than steel, but they are much stronger. Titanium is incredibly heat and pressure resistant, but considerably more expensive than steel for obvious reasons.

Bolt Carrier Finishes

Parkerized (Manganese Phosphate)

This type of coating is considered Mil-Spec and is probably the most common coating you’ll ever come across. It’s durable as much as it is highly resistant to corrosion. It tends to be one of the most economical bolt item choices.

Nickel Boron Coated

Nickel boron has the benefit of being extremely corrosion resistant and nearly friction-free. It’s convenient to clean, operates smoothly and can be cleaned simply by wiping it down with a soft rag.

Black Nitride

Black nitride is also seen as salt bath nitride or QPQ. It is not technically a coating, but rather a treatment that toughens and strengthens steel and makes it more resistant to corrosion. It also increases and improves longevity and reduces friction.


Testing proves just how robust and durable a BCG can be. Bolt carrier groups are often submitted to different kinds of tests as a quality control measure. Here’s a short explanation of what each test comes with.

Magnetic Particle Inspected

Magnetic particle inspection uses magnetism to find any flaws on the surface (or slightly below the surface) of a BCG. Here’s how it functions: the BCG is magnetized, and iron particles are placed on its surface. If there are any cracks, divots, or scratches, the particles will be taken to them. The test has no adverse effects on the BCG and promises a flawless product. From a lot of online firearm shops, you can look out for the description of a BCG to find out whether it has been submitted to magnetic particle inspection.

The Bear Creek Arsenal 6.5 Grendel Type II Bolt Carrier Group is one instance of a BCG that undergoes this type of testing after the manufacturing process.

High Pressure Testing

High pressure testing does exactly what its name says: it submits the BCG to very high pressures to see if they can counterattack the pressure of a live round. Some believe that this kind of testing is unnecessary and that it may shorten a BCG’s life span.