The Bolt Carrier Group is the very heart of the AR 15. Choosing a good one is an imperative.

by Paul Panella

Editor’s Note:  At SHOT our top reporter, Paul Panella, had the opportunity to acquire a BCG from a company that may not be a household name–Dadson’s Machining. Located in Hammond, Wisconsin, like many OEMs they do quality work but you simply don’t hear about them because they don’t cater to retail buyers. Dadson’s is one of those family owned companies that prefers to stay in the wings and let their work speak for itself. Paul decided it was about time to change that equation. Get ready to learn the ABCs of BCGs…

Whether you are buying or building your next AR15, it helps to know a little about what to look for in a quality bolt carrier group.

At the heart of your AR, the bolt carrier group contains the bolt, firing pin, and ejector. It performs multiple functions in a fraction of a second while taking the brunt of detonation force from your cartridge thousands of times. When considering BCGs it just makes sense to choose one that’s well made and is going to be durable while giving reliable performance.

At this year’s 2014 SHOT Show I had the opportunity to talk with Pete Cronin, Vice President of Dadson’s Machine, Inc. His company produces a variety of precision metal products for demanding industries from medical science to firearms that require top tier performance. Currently, Dadson Machine supplies AR parts to around 15 distributors/manufacturers of ARs, including bolt carrier groups. Pete explained Dadson’s take on what makes a better BCG and I’ve used one of their M16 style BCGs for comparison.

Some important things to look for:

  • Proper staking of the gas key
  • Fully shrouded firing pin
  • Steel Type
  • Manufacturing Process
  • MPI and HPT
  • AR15 vs. M16 style bolt
  • Coating

Staking

Staking is the process of peening or indenting the edges surrounding the hex screws that fasten the key to the bolt. This keeps the screws from backing out during thousands of rounds under recoil. Proper staking will have the edges impinging aggressively into the screws.

Good Staking 

A solid stake keeps the screws from backing out during thousands of rounds under recoil.

Light Staking

A lighter stake, without the proper indent, doesn’t provide the security that an operator needs.

Shrouded Firing Pin

A fully shrouded firing pin prevents the pin from being used as the initial component that pushes the hammer back to the fully cocked position during the extraction process. I haven’t seen an unshrouded pin lately and believe most makers have discontinued their use. They work, but the extra stress on a relatively fragile part (firing pin) is unnecessary and invites fatigue 

Steel

Currently, there are three steel types used in most bolt carrier groups:

Carpenter 158,  9310, and 8620.  (The steel type can be specified by the customer).

Carpenter 158 was the original mil-spec steel developed in the 1960s. But steel manufacturing has advanced the last 50 years and although Carpenter 158 is still mil-spec, new steels can be every bit as strong and durable. How each steel is manufactured, tempered, and machined will have a greater impact on the finished product than the AISI/SAE number.

Manufacturing Process

Poorly executed, even the finest ingredients won’t produce a good product. For instance, strength of steel is dependent as much on type as it is on process. Discovering just how dedicated manufacturers are to quality can take some investigating on the part of AR buyers. One step would be to find out who makes the BCG you’re thinking of buying and visiting the website or directly contacting the company.

Dadson’s Machine uses a vacuum consumable electrode process that forms the molten metal in a vacuum. This reduces free gas content and makes a denser finished product that is at least 99% solid. Less inclusions means less  chance of fracture or metal fatigue. The pieces are hot rolled and cold finished which improves strength and smooths the surface of the steel. Custom made state-of-the-art CNC machines pump out many parts in a single operation. Finished BCGs are completed in up to 2-3 less operations allowing less room for error.

MPI and HPT

Completed parts on the bolt carrier groups are High Pressure Tested for strength and then subjected to Magnetic Particle Inspection (aka MPI) for defects. The steel type is marked on the BCG along with the MPI assurance. Both these added processes are designed to insure reliability, durability, and strength by eliminating any surface or subsurface cracks, inclusions, or weaknesses.

Not all BCGs are equal in this regard.

An MPI Stamp is something you want to see on a BCG.

AR15 or M16 Style Bolt?

Aside from a fully shrouded firing pin, the most distinctive differences from one BCG to another can be seen comparing the amount of material milled from the rear of the bolt. Typical AR15 bolts have a smaller amount of steel on the tail of the bolt. This  style is used successfully on many of the ARs in production today including high end BCGs. On an M16 style bolt, the back of the bolt has more material. This adds more mass and adds nominally to the time it takes to begin extraction, slows the bolt/cycle time. Some argue that this slows wear and reduces muzzle climb.

The difference between the M16 and the standard bolt are clearly illustrated above.

Coating

The quest for incremental improvements to ARs is never ending.

Nothing wrong with the tried and true phosphate coating. Most of my ARs have it and they work just fine. I’ve also tried Nickel Boron (NiB), but there also are Black Phosphate, Ceramic coating, Titanium Nitride (TiN) to name just a few. All of them are supposed to reduce friction, make cleaning easier, and by extension, extend the life of your BCG.

Do they work?

Depends on what you’re looking for. As a recreational shooter, I’ve yet observe a big difference between BCGs no matter what the coating. If you’re a frequent competitor it might make a difference in longevity.

Conclusions

Considering how much punishment it takes and its importance in proper functioning of your AR15, the bolt carrier group ought to get as much attention as the barrel when choosing or building your new AR15.

The old adage: “Buy the best and only cry once,” applies.

If you’re interested in contacting Dadson’s Machining, you can contact Pete Cronin, Vice President at 715.796.2940

Photos courtesy of Paul Panella.

Questions? Comments?  

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