Microbest Military Grade M16 Bolt Carrier Group BCG - Nitride Exterior

MSRP: $139.00
Was: $108.00
Now: $99.99
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bcg-009.jpg Microbest Military Grade M16 Bolt Carrier Group - Nitride QPQ

Salt Bath Nitride Exterior Finish

The truth is, not all bolt carrier groups for the AR15 / M4 / M16 platform are created equal.  And, yes, there are only a handful of military grade BCG manufacturers.  Even a BCG from the same manufacturer can be made to varying levels of quality.  Why? 

Because very few of us, when shooting our rifle, need a BCG of the highest quality that our lives may depend upon it.  Most shooters will do well shooting with a BCG that is "good enough."

Well, here is the good news.  Charlie's and our sister company, Potomac Armory, went in search of the best military grade BCG, and worked with Microbest to design the specs to the best that can be.  No skimping on steel quality, cheaper hex bolts for the gas key.  All the best materials, and the highest grade quality testing:  High Pressure Testing and Magnetic Particle Inspection (HP-MPI) on a shot-peened Carpenter 158 bolt, and other upgrades.  Many brands that sell rifles to the US military depend upon Microbest, and we believe you can too.

The Bolt Carrier Group (BCG) is a critical component of the AR-15 (or M16/M4). The BCG is the “action” of the rifle. It loads a new round into the chamber, fires that round, extracts and ejects the spent casing, cocks the hammer and repeats the process.  The BCG is subjected to intensive wear, and with more people shooting shorter barrels and/or using suppressors, the stress on the bolt becomes even greater, and the more reason to have an excellent BCG.  Military grade BCGs are made for duty, action, and harsh environments of mud and sea water and sand and sweat.  While we all want to care for our bolt and carrier with a good oil, it also must function for days without proper lubrication.

The truth on Nitride BCG is twofold:  First, it is a much better finish than tradition chrome plated and phosphated BCGs.  The second is that the cost to product a nitride finished BCG is less than that of a phosphate and chrome lined carrier.  What this means is that many consumers equate less expensive to make with less quality.  It might take a generation for people to catch on to new technologies in firearms.  Nitride salt bath changes the chemical composition of the exterior of the steel.

Salt bath Nitriding/Nitrocarburizing is a process that is used to protect a metal surface and enhance its wear and corrosion resistance. During the nitriding process, a combination of nitrogen and carbon is diffused into a metal surface using a high-temperature salt bath. This permanently changes the molecular structure of the surface to provide a higher level of protection than plating or coating the material surface.

We all know chrome is very hard compared to steel, and traditional chrome lined carriers have a Rockwell Hardness factor of 67 to 70.  A QPQ Nitride finished steel has a Rockwell hardness of 80, so Nitride wins the contest, and there is a small amount of natural lubricity with Nitride finishes that means less oil is needed on the BCG.

And who would you rely upon to make a suburb BCG?  Microbest!



  • Nitride finished bolt and carrier for the greatest resistance to chemicals and metal on metal wear
  • Full auto profile 8620 steel alloy carrier, nitride salt bath finished
  • Hard steel allow gas key, fully nitride finished
  • HPT and MPI Tested
  • Carpenter Technology C-158 steel alloy bolt (shot peened)
  • 4130 Chrome Moly steel alloy gas key with staked Grade 8 hex screws and Loctite sealed
  • 4340 Tool steel extractor (shot peened)
  • S7 Tool steel ejector (shot peened)
  • Mil-spec stainless gas rings
  • 4340 Chrome Moly steel alloy cam pin
  • 8000 series steel alloy firing pin with nitride exterior
  • 1038 retaining pin in nitride exterior treatment
  • 15 ounce weight

Microbest is an OEM manufacturer for some of the best military grade bolts and carriers and full bolt carrier groups.  Those names and others.  Maybe some of the others do not demand High Pressure Testing or shot penning, or the most premium of bolts and steel allows.  Some of those names, like Colt and BCM sell for premium prices to have a letter stamped on the side, or a logo engraved, and the reviews for these brands always top the charts of the best available BCGs on the market.

Charlie's brings you the best of the best, and a price that is much lower that it should be.  Why?  Because you are not spending money on fancy packaging or brand marketing, or legacy names that command a premium.  And, because Charlie's and Potomac Armory has purchased in volume to bring a great savings to our customers.

The Microbest M16 Full Auto capable bolt carrier group ships full assembled, for your AR15, M16, or M4 with a Nitride exterior throughout the carrier, bolt and gas key. Machined to USGI specifications. This M16/M4 carrier includes the proper Mil-Spec gas key. Gas key is secured to carrier via USGI domestic manufactured upgraded Grade 8 fasteners and properly staked per Mil-Specs. These properly hardened parts ensure a strong staking and proper gas system functioning.

If you are looking to build an AR-15 or an M16 or M4 style military clone rifle for hard use, to standards that exceed Mil-Spec expectations, this Microbest BCG will fit that need. Every component of this bolt carrier group has been properly heat treated and is designed for reliability and longevity. The carrier is machined from AISI 8620 aircraft quality steel and features a full auto profile with durable manganese phosphate coating. The bolt is made from Carpenter Technology No. 158 Alloy and has been magnetic particle inspected as well as high pressure tested to ensure that it will run for many years to come. Every part on a BCG has a specific function and must be made to the highest standards, so that it doesn’t fail. The gas key is made from 4130 chrome moly steel.

This Potomac Armory selected Microbest BCG comes with a number of upgrades to the typical milspec BCG, like shot peening to help improve strength and even features small improvements like a precision ground gas key interface on the carrier to help secure the gas key in place. Even the cam pin has subtle improvements, like a solid film lubricant that helps reduce wear when the bolt is unlocking. The firing pin is hard chrome plated to prevent breakage. This BCG assembly will ensure that your AR-15 will run reliably for many years to come.

While this BCG is rated full-auto, it is not a restricted part, and this BCG will work just as well in an upgraded AR15 rifle, as it will in a military M4 or M16.

Form over function upgraded specifications

  • Lugs are machined with the proper radius as per true mil-spec tolerances. Independent HP white proof testing exceeded over 70,000 PSI without failure.
  • Outer diameter machined to all critical surfaces within +/- .0005 inches.
  • Rounded cam pin rides in a fully chamfered cam path.
  • Gas key with rounded leading edges ensures positive functioning in all makes and grades of uppers and buffer tube combinations.
  • Properly staked and torqued grade 8 hex screws as per the true mil-spec.
  • Carrier body OD and ID are hard turned after heat treat to +/- .0005 inches.
  • Multiple commercial finishes and coatings are available, engineered to their unique tolerances.


Any reputable manufacturer, Microbest included, will state their testing protocol for their bolt carrier groups. The tests determine whether the BCG can hold up under stress and pressure. Your BCG is the “heart” of your rifle – it needs to hold up. Generally, two tests are performed. Magnetic Particle Inspection (MPI) and High Pressure Testing (HPT). A quality BCG will be proof-marked with “MPI" or "HPT” or something similar if it’s been through the process. Microbest bolts are labeled MPI and individually tested for high pressure, and labeled for C-158 steel, which is one of the best steel alloys for bolts.


Magnetic Particle Inspection, or MPI, is where the bolt carrier group is magnetized and then temporarily coated with magnetic iron particles, usually themselves coated with a dye that reacts to ultraviolet light. The particles will group themselves in defects and cracks (if any) and glow intensely under the UV light.  The Microbest bolts selected by Potomac Armory are MPI inspected and so labeled.


High Pressure Testing, or HPT, subjects the BCG to pressures generally in excess of the usual pressure of the firing sequence. This overpressure tests to see if the BCG can hold up to sustained live firing.  Many brands will specify a sample to be high pressure tested.  Potomac Armory has specified that Microbest individually test every BCG, as if your life depended upon the bolt not breaking.


In so many ways, a proper M4 or AR15 bolt carrier group is the heart of a truly robust and reliable fighting rifle. Microbest provides OEM services to manufacturers which fulfill requirements for our armed forces. With Microbest components all meeting and exceeding military specifications, you can be assured that this bolt carrier group, selected by Potomac, will be the reliable core component of your AR-15 or M4 rifle.