FEATHER-WEIGHT BOLT CARRIER GROUPS: FAD OR RAD?

Written By Steven Kuo & David Lane

The AR-15 platform has arguably become one of the most finely tuned weapons ever to go bang. Its ubiquity, modularity, and open architecture have resulted in a veritable cornucopia of modifications and accessories designed to optimize it for a wide variety of applications.

Everyone from large established players to guys in their garage workshops with a good idea have conceived, produced, and marketed almost anything that you could imagine — including the topic of this article, lightweight bolt carrier groups (BCGs).

From machining lightening cuts to using space-age materials, these featherweight BCGs reduce the reciprocating weight in your AR-15, resulting in smoother operation and reduced felt recoil and muzzle movement — allowing you to get more hits in less time and to keep your eyes and optic on target throughout.

DIRECT GAS IMPINGEMENT 101

At its core, the AR-15 platform is a gas-operated semi-automatic rifle, specifically a direct gas impingement system. Press the trigger and the hammer is released, striking the firing pin. The firing pin ignites the primer of the chambered round, in turn igniting the powder and sending the bullet on its merry way down the barrel.

As the bullet travels down the bore, it passes by a small gas port. As this happens, gas hurtles into the gas port, through the gas block and back down the gas tube, toward the shooter.

The rearmost end of the gas tube is seated in the gas key of the bolt carrier group, so the gas continues into the interior of the bolt carrier (this is where the tail of the bolt assembly with its gas rings sits, hence all the fouling that builds up there) and pushes it rearward, cycling the action.

When moving rearward, the bolt carrier group pushes against the buffer and action spring contained within the receiver extension. The spring compresses sufficiently for the bolt to travel far enough back to extract and eject the spent cartridge and then strip and chamber a new round from the magazine (or alternately, lock back if empty).

This intricate dance is dependent on a number of factors to ensure reliable operation under varying conditions. The ammunition you're shooting might be hot military rounds, weaker commercial loads, or even custom hand-loaded mouse fart competition loads.

The relation of the length of the gas system (e.g. carbine, mid, intermediate, and rifle) to the overall length of the barrel determines how long the bullet remains in the barrel after it passes the gas port and affects how much gas is routed back into the action — as does the size of the gas port itself.

The combination of the bolt carrier group and buffer reciprocating against the action spring is also important.

Some complain that the AR-15 platform poops where it eats, but you still have to appreciate the ingenuity of Stoner's in-line design.

TUNING THE GUN

For applications where your life depends on your weapon, you would emphasize maximum reliability — so the mix of all of the components listed above needs to work no matter what type of ammo and what sort of conditions.

This could be hot military loads in the blazing desert, or weak imported commercial ammo in the freezing cold with a disgustingly fouled gun. The solution for this is typically plenty of gas and heavier components — the gun will kick more, but that's not the highest priority in this case.

Plus, work on your technique and you can enhance your recoil management.

But we're Americans, and we can't resist tuning things to optimize their performance, whether cars or guns. And when you have narrower operating parameters — known ammo, known range of conditions — you can do this with your AR-15 and still leave yourself with extra margin.

Because even if you're just shooting paper and steel for bragging rights or a prize table, you still want to win and a malfunction could be catastrophic to your rankings or ego.

Generally speaking you want to tune your rifle to operate reliably with the weakest ammo and coldest conditions that you expect to encounter.

Since the BCG (along with the buffer) comprises the mass that reciprocates from shot to shot, they affect felt recoil and muzzle rise. Reducing that mass — and also the gas operating the action — will dampen the first recoil impulse when the BCG and buffer hit the rear of the receiver extension as well as the secondary impulse when the BCG goes back into battery.

Utilizing your desired combination of adjustable gas block (or gas key), gas port size, gas system length, and ammunition, along with a lightweight BCG (and buffer), you can tune your rifle to consistently feed new rounds and lock back on empty. Standard buffers typically weigh from 3 to 5 ounces, while our baseline Mil-spec BCM bolt carrier group weighed 11.6 ounces — a lot to work with. Sure enough, the lightest BCG we tested clocked in at a feathery 5.74 ounces.

MATERIALS, FINISHES, AND TESTING

Steel

Many companies machine material away from the tried-and-true steel bolt carrier design to reduce weight. Some are more extensive than others, and the lightest are not able to retain the forward assist feature — some may bemoan this and others will celebrate it. 8620 is almost universally used, except for JP's 416 stainless steel offering.

They all boast advanced finishes as well (see Issue 13 for more details on the various coatings and treatments). While some tout that lubrication isn't necessary, we always use plenty of lube and enjoy the peace of mind that comes from the extra lubricity such finishes provide.

Titanium

A wonder material used widely in aerospace applications, titanium has the highest strength-to-density ratio of any metallic element, while being highly corrosion resistant. 6AL-4V titanium alloy is used by the companies in this lineup (note Boomfab did not disclose its material choice).

Compared to 8620 steel, 6AL-4V is about 40 percent lighter, with comparable yield strength, but lesser ultimate tensile strength. One could question the latter spec, but we'd argue that in the case of a bolt carrier, if it starts to deform it's already time to be concerned.

On the negative side, titanium is softer than steel and galls (which is adhesion and friction between two surfaces sliding against one another — sound familiar for a bolt carrier?).

To address this, all of the carriers are treated with some sort of surface hardening finish, whether physical vapor deposition, nickel boron, or H&M Metal Processing's brand new process (described later). In any case, you'll find us running titanium carriers even wetter than our steel BCGs.

Aluminum

Roughly 40 percent lighter than titanium, but with about the same loss in yield strength, aluminum carriers fall squarely into the realm of “race parts” to achieve the lightest possible recoil impulse. JP Enterprises and Whiskey Arms both advise their products should only be used in competition rifles, not duty or defensive weapons.

Their carriers are both hard anodized, bringing aluminum to the approximate equivalent in excess of 60 on the Rockwell C hardness scale. But it's still aluminum underneath, with hardened steel scraping against it every which way. JP's instructions state, “If you don't have oil spraying on your shooting glasses, you need more oil.” Nuff said.

Beyond lightening, most also modify the BCG spec, whether increasing or tweaking the bearing surface area or changing the gas ports and cam pin path.

Gas keys were all aggressively staked to prevent the screws from moving. All bolts are steel, either C158 or 9310 depending on the company.

All are also shot peened to reduce susceptibility to metal fatigue.

Testing procedures during manufacture and quality control have gained visibility of late with the consumer market's appreciation for all things Mil-spec.

All of these manufacturers perform magnetic particle inspection (MPI) to detect surface cracks. Some companies also high-pressure test (HPT) their bolts, proof testing them under high pressure to check for failure.

While some folks have religion about this, we don't — with certified high-quality materials from reputable manufacturers, we sleep soundly at night, knowing that life expectancy will also be longer than those subjected to HPT.

Without further ado, here's a wide selection of the best the industry has to offer, from aluminum to titanium to lightened steel carriers.

There's bound to be a product that has the exact mix of characteristics and features that you're seeking..

MIRROR, MIRROR ON THE WALL, WHO IS THE LIGHTEST OF THEM ALL?

Because we're, well, RECOIL and we do OCD shit like this, we obtained a calibrated laboratory scale to precisely measure the bolt carrier groups. The Whiskey Arms BC doesn't come with a bolt, so for the sake of comparison, we've presented their total weights here with a standard bolt assembly with a titanium firing pin. For our curious, detail-oriented readers, a titanium firing pin saves approximately 0.03 ounces of weight.

BEST LIGHTWEIGHT BCGs

JP Enterprises Ultra LMOS Bolt Carrier Group with JP EnhancedBolt

JP Enterprises has long been ahead of the curve, offering complete competition rifle systems with lightened components and adjustable gas before it became trendy. It actually used to sell aluminum BCGs over a decade ago, but stopped because customers weren't maintaining them properly and complaining about premature wear.

With lots of prodding to bring them back, JP complied with the lightest BCG on the market. Bearing surfaces on the carrier are substantially larger than standard BCGs. Comes with a titanium firing pin to eliminate potential for slam-fires and a one-piece gas ring.

Available with or without JP's EnhancedBolt with radiused bolt lugs.

• Weight (complete BCG / carrier only): 5.74 / 3.77 ounces
• Carrier Material and Finish: 7075-T6, anodized
• Bolt Material, Testing, and Finish: 9310, proprietary, chromium nitride
• Price: $461

Whiskey Arms LBC-Aluminum Lightweight Bolt Carrier

During the time that JP had discontinued its product, Whiskey Arms' owner longed for a super-light aluminum BCG.

So he set out to make his own at the most affordable price point that he could hit. The result is the LBC, offered as a carrier only (with gas key installed) and intended to replace the full-weight carrier on your existing BCG while reusing your bolt.

Of course, you can also pair it with a bolt from your favorite provider (it's shown here with a NiB bolt assembly). Bearing surfaces are enlarged with some relief cuts. Note that you lose forward assist capability.

• Weight (complete BCG / carrier only): n/a / 3.89
• Carrier Material and Finish: 7075-T6, anodized (oil impregnated)
• Bolt Material, Testing, and Finish: n/a
• Price: $235 (bolt not included)

Mystic Black Titanium BCG

Offering the same magic coating as their normal BCGs, Cryptic Coating Mystic Black Titanium BCG is an expensive option — but if weight savings and ultra-low friction coefficient is your goal, this might be the best there is.

The bolt carrier is machined from Aircraft Quality 6A1-4V Titanium with a Carpenter 158 bolt with spring and ejector.

• Weight (complete BCG): 7.78 ounces
• Carrier Material and Finish: 6AL-4V, proprietary CVD
• Bolt Material and Testing: Carpenter 158, MPI & HPT
• Price: $450

Rubber City Armory Titanium Complete Bolt Carrier Group with adjustable gas key

RCA's sister company, H&M Metal Processing, recently developed a new patented thermal chemical diffusion process to treat titanium and titanium alloys for high-wear applications.

Like the company's nitriding process for steel, it's not a coating on top of the substrate material, but rather a treatment affecting the surface of the metal. Intended for aerospace applications, it also found a great application in lightweight BCGs (the prototype is shown here).

RCA says it can endure abuse like nobody's business, so we look forward to running it hard. It's available with a standard or adjustable gas key, so you can fine-tune uppers lacking an adjustable gas block.

• Weight (complete BCG / carrier only): 7.8 / 5.65 ounces
• Carrier Material and Finish: 6AL-4V, nitride
• Bolt Material, Testing, and Finish: 9310, MPI, Blacknitride+
• Price: $380

JP Enterprises LMOS Bolt Carrier Group with JP EnhancedBolt

The elder statesman of the group, JP's LMOS system has been a staple for competition shooters for years — and is still the second lightest of all the steel BCGs in this group.

The LMOS BCG displays all the maturity and refinement you'd expect from JP. Available in various configurations and for .223, 6.5 Grendel, and .308, it's pictured here with JP's EnhancedBolt. The bearing surfaces are significantly larger than standard, and it's finished in either a beautiful mirror polish or black QPQ nitrided.

• Weight (complete BCG / carrier only): 8.77 / 6.68 ounces
• Carrier Material and Finish: 416 Stainless steel, polished or QPQ nitrided
• Bolt Material, Testing, and Finish: 9310, proprietary, Chromium nitride
• Price: $353

Rubber City Armory Low Mass complete BCG with adjustable gas key

The third lightest steel carrier in this bunch, the RCA offering features its popular nitride finish and an optional adjustable gas key.

Utilizing set screws that protrude into the gas key, you can meter the gas delivery — very handy as a turnkey drop-in system if you want to use an upper that already has a fixed gas block. The bearing surfaces are also slightly larger.

Note that the standard gas key on our sample shown above was replaced with the adjustable one and rushed out to us, and it inadvertently wasn't staked before shipment — their production units are all staked.

• Weight (complete BCG / carrier only): 8.9 / 6.94 ounces
• Carrier Material and Finish: 8620, Blacknitride+
• Bolt Material, Testing, and Finish: 9310, MPI, Blacknitride+
• Price: $269

Spikes Tactical Lightweight Nickel Boron HPT/MPI M1 Bolt Carrier Group

Sporting Spike's Tactical distinctive flair, this BCG is perforated with ovals for a modest reduction in weight (it's the heaviest steel BCG in this group).

Coated throughout with nickel boron, the BCG is otherwise the same as the company's Mil-spec offerings. It has slightly larger bearing surfaces and comes with an extractor insert and O-ring.

• Weight (complete BCG / carrier only): 10.5 / 8.37 ounces
• Carrier Material and Finish: 8620, Nickel Boron
• Bolt Material, Testing, and Finish: C158, MPI/HPT, Nickel boron
• Price: $250

Faxon Firearms Gunner Lightweight

If you're a fan of Faxon barrels, this might be a no-brainer for you. Faxon, while known for their barrels, also makes a fairly large range of other AR and pistol parts — one of those is their Gunner Lightweight BCG.

It might look fairly normal and what you might expect from a steel low-mass BCG, but it has a few tricks up its sleeves.

Off the bat, Faxon uses what they call a “superfinish” on the body of the bolt carrier. This isn't just marketing fluff, this is a much finer and well-polished finish that reduces the friction of the carrier and makes it significantly slicker and smoother.

Plus, this is simply a quality BCG. Salt bath nitride, mil-spec materials, and a pretty decent price point.

• Weight (complete BCG): 8.5 ounces
• Carrier Material and Finish: 8620, Salt Bath Nitride w/ “Superfinish”
• Bolt Material, Testing, and Finish: 9310, MPI, Salt Bath Nitride
• Price: $250

Brownells Lightweight BCG

If you want to go ultra-cheap, then Brownells is your answer. Coming in at a weirdly low price point, the Brownells BCGs don't get the respect they deserve (in my opinion).

Honestly, there is nothing fancy about this BCG. It's steel, it's skeletonized, and it's Nickel Boron coated.

Or, if you want, Brownells offers these with a TiN coating or just Nitride. The TiN is the same price, and the Nitride is only $120.

I've run the NiB BCG long and hard and it stood up to my abuse, so that's the one I'll recommend to you. Maybe Brownells isn't the coolest or most innovative, but it's well-made and cheap. If you're just looking to dabble with the low-mass BCG idea, this is a perfect stepping stone.

• Weight (complete BCG): 8.2 ounces
• Carrier Material and Finish: 8620, Nickel Boron
• Bolt Material, Testing, and Finish: 9310, MPI, Nickel boron
• Price: $150

LOOSE ROUNDS

Lightweight BCGs aren't for everyone or every rifle, but if you're willing to accept the compromises, there are some awesome BCGs out there to try.

Ready For More?

• Best AR-15 Bolt Carrier Groups (BCGs) [2022 Buyer’s Guide]
• The SCAR Without FN: Building A Scar At Home
• Best AR-10 Complete Upper Receivers

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