Bolt Carrier Groups are made up of several parts. The bolt assembly which interfaces with the barrel extension and feeds and extracts the round from the chamber, the carrier which holds the everything together and creates rotation of the bolt, and the gas key which rests against the gas tube and “Catches” the gas from the tube when a round is shot thus operating the action. The Bolt Carrier Group also contains the firing pin which the hammer strikes upon trigger pull to fire the round. This bolt carrier group is a critical component of the AR15.
Bolt carrier groups can have differences in weight based on their intended purpose. For example a “full auto rated” M16 carrier will be approximately 1 oz. heavier than a standard AR15 civilian semi auto bolt carrier. There are also light weight “low mass” bolt carriers on the market. These weight differences are partially to control cyclic rates. All things being equal, the lighter the bolt the faster it should cycle. However the low mass bolt carrier are almost always used in conjunction with AR15’s with tuned or adjustable gas systems. The heavier M16 style bolt carrier groups are commonly used in semi auto AR15’s without negative result, and are often specified in carbine length gas systems.
The coatings available on the bolt carriers can seem like a near endless list. Which one is better, and which one does what? A standard bolt carrier group is phosphate coated. Chrome bolt carriers have been on the market for quite some time but there are other coatings such as TiN, NiB, NP3, and on and on. What do all these coatings do?
Phosphate coating, also called Parkerized, changes the surface of the metal making it more porous. This porous surface helps the metal absorb lubricant more effectively. A phosphate coated Bolt Carrier groups is the standard Mil-Spec.
Chrome Bolt carriers are chrome plated steel. The chrome plating helps with corrosion resistance, especially in adverse or humid climates. It also strengthens the surface making the surface more resistant to wear, though the wear on a bolt carrier group is actually somewhat negligible to the average shooter.
Titanium Nitride (TiN) and Nickle Boron (NiB) are two new flavors of coatings provided on bolt carrier groups. Each functions very much like the chrome plated bolt carrier, however create either smoother or tougher surfaces depending on molecular structure. These chrome, TiN and NiB coated carriers generally are also easier to clean and require less lubrication when compared to standard phosphate coated Bolt Carriers.
Which coating is right for you? Generally speaking a tactical rifle will be phosphate coated, and you can’t go wrong with any AR15 rifle having a phosphate coated bolt carrier group. The phosphate coating process also creates a dull black surface perfect for tactical AR15’s where shine can be a bad thing. Hunters or those who need superior weather resistance or those who don’t like cleaning their firearms often, may opted for a chrome TiN or NiB coated bolt carrier.
High Pressure (HP) testing and Magnetic Particle (MP) testing are two quality control tests performed on BCG’s during the manufacturing process to determine if there are any surface defects in the parts. MP means the parts were magnafluxed (basically, x-rayed) for cracks/issues. MP is not as good without HP, when a “proof” load is fired before the magnafluxing. These tests are a good thing to have but since they won’t show you all the potential defects one must consider that any BCG is prone to failure over time. A HP/MP tested BCG is slightly less likely prone to failure because it will have no apparent surface defects. Some shooters won’t buy a BCG without knowing these tests have been performed, while in most cases civilians would be fine with untested parts. Often BCG’s which have been HP or MP tested will be stamped.
There is some debate within the AR15 community regarding the meaning of MP. Some regard MP to mean: MAGNETIC PARTICLE TESTED / PROOF LOAD FIRED. If you are concerned with this, the best thing to do is contact the manufacturer of the bolt.