The AR-15 is America’s Rifle. An estimated 25 million Americans own one or more of these versatile firearms. The total number of AR-15s in the country is difficult to determine, but some reports claim 44 million or more. Regardless, it’s clear that the AR-15, like it or not, is astoundingly popular. One of the reasons the AR-15 has achieved a near-ubiquitous presence in the firearms world is that the rifle is extremely modular, meaning the main parts and assemblies are easy to assemble and swap out, so owners can acquire parts and components from multiple companies, do any necessary FFL paperwork on a serialized lower receiver, and put them together themselves, in various configurations. Some people jokingly call this hobby Legos for grown-ups.
One of the features that has made the AR-15 so popular is that unlike with many rifles, AR-15 barrels, bolt carrier groups (BCGs), handguards, receivers, trigger groups, and stocks are usually interchangeable among different brands, and the bolt/barrel extension almost always headspaces safely and correctly without using any special tools or gauges. Suppose all the components are manufactured within the original dimension tolerances. In that case, you can purchase a lower receiver from one manufacturer, an upper receiver from another, a barrel, BCG, trigger group, parts kit, receiver extension/buffer tube, and furniture from others, and the rifle will almost always go together without issue and function as designed. This is extremely uncommon in the world of centerfire firearms.
A huge aftermarket industry has developed and now offers multiple options for all AR-15 components and parts, in addition to companies that make complete rifles. Today, dozens of companies provide BCGs for sale, either to people assembling their own AR-15 rifles or those wanting to upgrade to better components as they go. Let’s go over what makes the BCG the heart of the AR-15, and then we’ll briefly give some of our recommendations for finding a good one.
Understanding AR-15 bolt carrier groups (BCGs)
The most crucial component of an AR-15, in terms of reliable function, is the bolt carrier group. Every action of the rifle, from chambering a round, firing the cartridge, extracting the fired case from the chamber, ejecting that case, and stripping another round from the magazine, depends on the BCG.
On an AR-15, the bolt is what interfaces with the barrel extension and locks the rifle into battery for firing. When locked into battery, the seven lugs on the bolt are rotated to engage with the lugs on the barrel extension. As the trigger releases the hammer, the hammer impacts the firing pin contained in the BCG. The cartridge fires and the bullet passes the gas port in the barrel, and gas is bled off, traveling back along the gas tube, into the gas key or carrier key on top of the BCG, and into the pressure chamber of the BCG behind the bolt. This forces the bolt forward against the chamber and removes load from the bolt lugs. As pressure drops, the bolt cam, traveling in its angled track on top of the BCG, rotates the bolt, unlocking its lugs from the barrel extension lugs. The bolt carrier then begins to move rearward, with the bolt acting as an internal piston that forces the entire BCG to the rear. The hammer is recocked by the BCG. The spring-loaded extractor claw grips the rim of the case, pulling it from the chamber, and the spring-loaded ejector in the left portion of the bolt face pushes against the left side of the case, pivoting the case out the rifle’s ejection port to the right. Then the cycle repeats.
A poorly made or improperly machined bolt carrier group can cause all kinds of problems. Bolt lugs can shear off or crack, firing pin retaining pins can break, bolt cam pins can fail, the rear of the bolt can break off, extractors can break, and gas keys can crack or work loose. The passage in the BCG for the bolt tail must be properly sized to seal too much gas from traveling rearward through the BCG, or malfunctions can occur. All of the critical surfaces on the inside and outside of the bolt carrier must be within proper spec and sufficiently hardened to prevent excess movement of the bolt and gas leakage, resulting in cycling issues. This is all to say that the BCG is not something that you want to cheap out on when considering purchasing a rifle or assembling one from parts.
Here’s a helpful video using cutaway parts on a demonstration rifle so you can get a better idea about what’s going on as an AR-15 cycles. It also covers in detail how important all of the tolerances inside the BGC are to proper, safe function of the rifle system.
Video: AR-15 Bolt Carrier and Rifle Cut Away Demonstration
What does Mil-Spec actually mean when discussing AR-15s and parts?
Mil-Spec, or sometimes MILSPEC or MIL-STD (military standard), is a very commonly misunderstood term associated with AR-15s and their parts, including BCGs. Mil-Spec is short for Military Specification, and refers to requirements and acceptance documents produced by the US military for every piece of equipment. To understand every aspect of the term Mil-Spec when it comes to an AR-15 would require volumes. While some parts of the civilian, semi-automatic AR-15 may interchange with the select-fire or burst-fire military M4 or M16, many do not, so may not accurately be described as Mil-Spec, though they frequently are for marketing purposes. Furthermore, to be absolutely specific, a truly Mil-Spec part or assembly must be able to be placed into the US military arsenal without any variation from the actual military component. But the short answer is, for real-world purposes, companies who produce products that conform to Mil-Spec dimensions and materials can help buyers assemble components that have a better chance of functioning properly together and remaining durable according to military standards.
One possibly confusing point is that Mil-Spec doesn’t necessarily mean better. Often terms like MILSPEC or military grade are used to market lower quality items, because many people assume the US military requires objectively superior equipment. This is frequently not the case. In other cases, the actual Mil-Spec requirements are either outdated or objectively inferior to other options. Just as one example, a Mil-Spec M4 barrel must be chrome-lined in accordance with accepted processes and tolerances. Properly manufactured chrome lining can help increase corrosion resistance and improve durability, but it also can lower the accuracy of the rifle, when compared to quality non-chrome-lined barrels. If your goal for your AR-15 is to achieve the utmost accuracy, going with Mil-Spec chrome lining can actually be a detriment.
Additionally, there are modern coatings and processes that can produce metal surfaces with greatly enhanced performance compared to the Mil-Spec parkerized/phosphate coated bolt carrier group. But improvements and updates to the Mil-Spec document/requirements can take years or even decades to make it through the government/military bureaucracy and be implemented. Additionally, military contracts are frequently awarded to the lowest bidder. So, while Mil-Spec may offer you some assurance of basic quality and functionality, you should do your research and take any claims of superiority of Mil-Spec AR-15 components with a grain of salt.
Full-auto vs. commercial BCGs
A full-auto bolt carrier has a bit more meat at the lower rear portion than a commercial or civilian bolt carrier. This extended portion of the carrier is designed to trip the auto sear on a military M4 or M16, enabling fully automatic or burst fire. Civilian or commercial AR-15 lower receivers don’t have the provision for the military full-auto trigger group, and modifying an AR-15 to contain these key parts is both difficult (impossible in some cases) and highly illegal, without the proper federal weapons manufacturing licenses.
However, using a full-auto or full-profile BCG in a civilian AR-15 is not illegal, and is perfectly fine functionally. There’s no full-auto sear to trip, so the more-forward cut in the carrier has no real purpose in this case, but does no harm. Many AR-15s are sold with M16-profile BCGs installed, and some may argue that the slightly heavier bolt carrier (due to the longer lower section that hasn’t been milled away) slows bolt velocity a little and can improve durability over the life of the rifle. We’re not aware of any credible testing demonstrating this claim, but most experts have no problem using either quality commercial-profile or full-auto-profile BCGs in their rifles.
Best AR-15 bolt carrier groups
As noted above, the BCG is the heart of the AR-15. If you were to pick one or two components of the rifle that contribute to reliable function and accuracy, it would without question be the bolt carrier group and the barrel. You could install a top-quality barrel and BCG on even the least expensive receiver set (assuming the upper and lower receivers are within design spec) and the rifle would most likely be extremely accurate and reliable. There are dozens of choices for AR-15 BCGs today, though many of them are actually made by the same 3 or 4 manufacturers. Let’s take a quick look at some of our top picks in several categories.
Best Mil-Spec BCG for the money: Toolcraft Phosphate
Toolcraft manufactures quality AR-15 BCGs for multiple brands, in several configurations and coatings. The greatest bang for the buck is their standard magnesium phosphate BCG, available from places like Palmetto State Armory for around $99 most of the time. The full-auto profile carrier and the bolt are finished in magnesium phosphate, also known as Parkerization, for good oil retention and corrosion resistance. The bolt is Magnetic Particle Inspected (MPI), shot peened for hardness, and made of Carpenter 158 steel, per MIL-SPEC. The hardened carrier gas key and carrier are both chrome-lined, and the key is secured with Grade 8 bolts and properly staked. If you’re looking for a quality Mil-Spec AR-15 BCG for a great price, it’s hard to do better than Toolcraft. Note: Toolcraft also offers its basic BCG in a very hard, very slick DLC (diamond-like carbon) finish for $174 MSRP, available for around $159 from several retailers.
Other go-to Mil-Spec BCGs you can rely on: BCM MPI Auto BCG ($199); AO Precision Premium Phosphate BCG (prices vary, but usually around $99).
Best hard chrome BCG: Young Manufacturing
With properly applied hard chrome, fouling just wipes off, and you may find you need far less lubricant on the bolt and carrier than with legacy coatings and finishes. Hard chrome is difficult to apply properly, but Young Manufacturing does it right. For quite a few years, Young was the only firm offering a hard-chrome finished BCG. Today, Young Manufacturing offers multiple configurations of AR-15 BCGs, including their very popular National Match enhanced carrier and bolt, with precision machined tolerances to help ensure a more consistent lockup for improved accuracy. If you want a Mil-Spec style BCG but with the improved lubricity, wear resistance, and ease of cleaning of a hard chrome finish, their standard M16 Chrome BCG ($274 MSRP) is a solid buy.
Young also offers their BCGs in a black nitride finish (which is superior to Mil-Spec phosphate but not as expensive as hard chrome) for $139. You can also opt for their HMB bolt design for roughly a $40 upcharge. The HMB bolt by HM Defense eliminates the pass-through cam pin hole and replaces it with a cam pin socket and radiused cam pin design. The new design eliminates the natural weak point of Mil-Spec bolts by significantly increasing the amount of metal (and strength) at the cam pin location. The HMB Bolt also reduces contact between the cam pin and firing pin, enabling smoother operation of the firing pin, bolt lugs and gas rings. An additional benefit of the cam pin socket is that it retains more lubricant compared to a pass-through design, which may improve performance over long periods of operation.
Want to try a chrome BCG for less? Microbest ($139) has a good reputation.
Best enhanced AR-15 bolt design: Knights Armament E3
This is one example where strictly Mil-Spec parts can be improved, and have been. Knights Armament Company (KAC) is well-known for top-tier military firearms, and their SR-15 is an enhanced AR-type rifle with several improvements over the run-of-the-mill. One of the primary differences is a new, enhanced E3 bolt design with rounded lugs, rather than the standard rectangular lugs. This requires a different barrel extension as well, and an E3 bolt can not be used in a Mil-Spec, standard AR-15 unless you also swap the barrel extension to a Knights design. However, for people who shoot a lot, particularly short-barreled and/or full-auto rifles, and often using suppressors, the improved reliability and durability of the E3 bolt (which sells for around $275 just for the bolt itself) is well worth the upcharge.
Video: The SR-15 and the E3 Bolt - KAC TV - Trey Knight Explains
The enlarged bolt face provides more positive engagement in the chamber and ensures proper seating as the BCG cycles. The rounded bolt lugs are less prone to chipping or cracking than traditional squared-off bolt lugs, due to fewer stress risers at the corners. A reduced-diameter cam pin thickens the bolt web, strengthening one of the weak points in the AR-15 bolt. The extractor is also improved, with a widened base acting on two separate springs for consistent tension and stability. The extractor’s pivot point is moved forward for a more constant grip on the case rim for reduced extraction failures. There’s also an extractor pin retention spring to keep that pin from walking out under hard use. With KAC you’ll pay a lot, but you get a lot in return.
Best tuneable/recoil-reducing BCGs: JP Enterprises
JP Enterprises has long been known as the go-to shop for high-performance race guns for 3-gun competition and other shooting sports where maximum speed and minimum recoil are the priority. These are typically not rifles that will be used for combat, defense, patrol, or any other serious use, but rather specially tuned race guns intended for peak performance under the pressure of the shot timer. In combination with an adjustable gas block, aggressive muzzle brake, and ideally the JP Silent Captured Spring and buffer system, you can achieve a rifle with effectively zero recoil in some cases.
The ultimate in low-mass, low-recoil BCGs is JP’s 7076 T6 aluminum Ultra LMOS low-mass bolt carrier, starting at $365, or $485 including their EnhancedBolt. An aluminum BCG may sound like insanity, and indeed their original design was discontinued due to users not adhering to the more stringent maintenance requirements. However with an improved aluminum material and coating, JP has opted to bring back the ultra-low-mass BCG, weighing just 3.76 ounces with the carrier key installed… less than half the weight of a standard BCG. You’ll definitely want to pair this with an adjustable gas block, effective muzzle brake, and JP’s Silent Captured Spring (starting at $139) to ensure that the timing of the bolt unlocking/cycling process is within operational limits. However, when you get all of that right (and it can be kind of a dark art), you can experience an AR-15 that recoils like a .22LR. Note that aluminum will never wear like steel, so in a head-to-head durability contest, an aluminum carrier will not win. Take that under advisement when you opt for this super-specialized, super-lightweight BCG.
JP offers a Low-mass BCG in stainless ($245; 6.68 ounces with the carrier key installed) for greater durability, as well as a full-mass 416 stainless BCG ($245; 8.82 ounces with the key installed) for customers desiring a more widely reliable rifle system at the expense of a little more felt recoil. For the ultimate in tunability, particularly for people wanting to shoot suppressed, the VMOS variable mass carrier ($335; or $455 with EnhancedBolt) offers swappable ring weights at the rear of the assembly that, when combined with the Silent Captured Spring, allows multiple combinations of mass to perfectly tune your operating system for recoil reduction and reliability. Note that the VMOS requires the use of the Silent Captured Spring system, as a mil-spec buffer retaining pin will interfere with the ring weights in the VMOS, causing malfunctions and damage.
Other options for a tuneable BCG: Sharps Rifle Company Adjustable Gas Recoil System ($247). This DLC-coated BCG takes a different approach. Rather than using a combination of weights, the design allows the user to restrict or open the flow of gas into the gas block using an allen/hex wrench and a screw at the rear of the carrier key. Combined with the proper buffer weight, recoil and excessive gas into the shooter’s face are reduced, particularly when shooting suppressed. Walker Defense also offers a reduced-weight titanium BCG with a nickel-boron coated bolt for $359.
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