Enhanced Bolt Carrier Groups

Enhanced Bolt Carrier Groups

ABOVE: As designed at the request of SOCOM, the Lewis Machine and Tool Enhanced Bolt Carrier Group. Note the addition of an additional gas port.

Anyone who has been heavily involved with maintenance and repair of the M16/M4 rifle weapon systems knows that there is room for improvement with the bolt carrier group. In the mid 1990s, SOCOM reported many failures of bolt fractures at the cam pin slot on the bolt, broken locking lugs (usually lugs 1 and 7) as well as failures to extract and eject on the M4A1 carbines. There are many reasons for these malfunctions; everything from manufacturing errors to ammunition issues have contributed to the condition and in many cases, just the operating dynamics of the shorter gas system speeding up the cyclic rate and the bolt unlocking and trying to extract a cartridge case that is still slightly expanded.

These issues led to two different actions. First Karl Lewis of Lewis Machine & Tool was contacted to come up with an enhanced bolt carrier group that would deal with these conditions and the second was to come up with an upgrade kit that would aid in repairing these issues. This would include a new 1-piece Mcfarland gas ring, new heavy duty ejector spring and a rubber “O” ring that would slide over the extractor spring assembly that would greatly increase the extractor force. This would go on to be standard issue on Mk11 and Mk12 rifles.

Shown is a LMT Guardian with M203 grenade launcher. Note the Smith Enterprises chrome bolt carrier. The AR-10/AR-15 started out with the use of a chrome plated bolt and carrier.

On April 28, 2011, Solicitation Number W15QKN-11-R-F002 was issued for an improved bolt carrier group that would deal with almost all of these issues for general issue – to replace the bolt carrier groups at large with a better option. Several companies took a shot at this task and submitted. Some had dealt with some of these issues but did not submit. In the end, army testing concluded that none of the enhancements “significantly” enhanced the existing group and the project was cancelled. Very similar to the recently cancelled Individual Carbine program, it too did not have enough of an improvement to warrant a change. Many experts disagree with the Army including this author. Several enhancements were made that directly addressed and corrected these issues and would in fact have made the rifle more reliable and durable. But the question is what exactly constitutes “significant” improvement?

The Lewis Machine & Tool Enhanced Bolt Carrier Group

The first to be asked and the first on the scene was the Enhanced Bolt Carrier Group (BCG) by LMT that addressed all of the issues. Beginning with the bolt carrier itself, several modifications were made with the most significant being an alteration of the cam pin track that would allow more dwell time. In other words, it would keep the bolt locked slightly longer allowing for more residual pressure to drop that resulted in easier extraction. Also, the gas port on top of the bolt carrier directed gas from the gas tube/carrier key to two sides rather than dump it all on top of the tail of the bolt and rear of the bolt carrier (piston chamber). This also slowed down the introduction of the gas that also led to an increase in dwell time. The location of the exhaust port on the right side of the carrier location was moved forward and upward. This accomplished two things. First it regulates peak pressure to further slow down the action and second it released the gas sooner in the cycle so it would exit out the ejection port instead of inside of the upper receiver. There was a third vent added to the carrier to allow quicker egress of unused gas. LMT added a flange to the face of the bolt carrier to fully support the bolt and strengthen the carrier at the end where the dwell was increased, which decreases any chance of cracking. Sand/dirt relief grooves were added to the top and bottom of the front portion of the bolt carrier. These grooves channel away any debris in the upper receiver to allow the carrier to move smoothly.

The LMT Enhanced Bolt Carrier Group has many enhancements. Note the stress relieved bolt lugs as well as the additional gas vents on the carrier.

The bolt was also highly modified. Built from a much stronger material from the start, this bolt is more expensive to manufacture due to the higher quality material and more expensive to produce due to tools wearing sooner from cutting the harder material. The cartridge case base is fully supported on the LMT bolt contrary to the cut out of material, which is where the extractor fits onto the bolt on the standards. The extractor has been completely redesigned. The “lobster tail” extractor utilizes two long extractor springs cut on a slight angle that give much more extractor force and the springs last significantly longer than the standard. The standard spring has very little room for improvement due to the short spring compartment between the bolt and carrier. The use of two long springs is a great enhancement. The extractor claw has been redesigned to grip more surface of the cartridge case rim as well as the hook is much stronger than the standard. The bolt has a proprietary nickel-type finish to aid in function and to make cleaning easier. The bolt is both proof tested and magnetic particle inspected to insure against stress fractures in the material. The locking lugs have rounded edges instead of squared off edges.

This bolt carrier group addressed all concerns but was never adopted by SOCOM or the Army. Perhaps because it would not interchange with the M16A2/M16A4 rifle. The bolt carrier would cause light strikes on full auto with the full length barrel. The Enhanced BCG was designed specifically to improve the performance of the M4 carbine and tuned to work with its dynamics. The army is very much entrenched in compatibility of components. However, this remains perhaps the finest enhancement available for the platform.

LWRCI International

The LMT Enhanced Bolt. Karl Lewis is the designer of the “Lobster Tail” dual spring Extractor. Also this was the first bolt to have a fully supported cartridge case base.

The BCG submitted by LWRCI took a novel approach to the problems as well. The bolt carrier is very well thought out and does something none of the others do and that is that it eliminates the possibility of gas leakage due to a loose bolt carrier key or broken carrier key screw. LWRCI took the tombstone design of their piston carrier and added it to their DI (direct impingement) carrier. They drilled a vertical hole to make the gas channel into the carrier tombstone. They then designed a linear gas key that would screw into the face of the tombstone. The key would index by two markings when the key was aligned with the gas port and then pinned in place and secured. This excellent design eliminates gas loss from a damage carrier key or screw. The bolt carrier is media blasted to ensure a non-reflective finish and then case hardened for a wear resistant component with a tough core. The bolt carrier has forward assist notches. There is a space between the running surfaces of the carrier and the inside of the upper receiver to allow the working parts to push and collect debris and prevent them from stopping the weapon. The running surfaces of the bolt carrier rails, or sand cuts, allow the bolt carrier to plow through sand, mud, dust and ice preventing malfunctions. Flat cuts on the sides of the carrier provide space for the plowed material to be pushed so not to interfere with the operation of the weapon. The carrier also features a corrosion resistant electroless nickel with boron finish, which is extremely wear resistant, corrosion resistant, reduces the co-efficient of friction between the bolt carrier rails and the upper receiver of the weapon. The addition of Teflon suspended in the matrix of the plating increased lubricity between other parts such as the bolt, cam pin and hammer. This finish does not require wet lubricant but is still suggested in small amounts. This finish also makes the carrier easier to clean. The firing pin retainer pin has been modified to insure longevity. Instead of the standard round hole, a counter sunk “T” shape is used. This allows the retainer pin to be inserted in its most strong position and prohibits the pin from being rotated and being bent.

LWRCI also submitted with their bolt carrier a proprietary bolt design as well. Called the Advanced Combat Bolt, the bolt is made from 9310 steel-alloy bar stock and then carburized for case hardness and then tempered to increase core toughness and then shot peened the surface of the bolt body to induce compressive stress and improve fatigue life. The bolt was designed with improved fully supported locking lugs. Due to the most common lug failures on the standard being lugs 1 and 7, the lugs experience greater load due to the spacing between them and the lack of a lug. The supporting material is cut away from these lugs to accommodate the extractor pocket. The LWRCI bolt has been designed so the extractor pocket has been redesigned allowing the two bolt lugs to remain fully supported by the bolt without affecting the extractor. All the corners have been rounded rather than square cut removing stress risers and concentrators. The bolt face on the LWRCI carrier fully supports the base of the cartridge case head as opposed to the standard bolt face that is partially cut out to allow the extractor pocket. By fully supporting the case head it diminishes the likelihood of a case head failure. There is also a “crud” groove around the outer edges of the case head that allows brass shavings and loose primer sealant a place to go rather than cause fouling in the weapon. The bolt also is designed with an improved “whale tail” extractor. The cam pin slot has been modified to prevent cracking in this location, which has been seen on standard M1 and M4A1 carbines. The hole is counter-bored and will not stress the bolt causing micro-fractures and far more consistent than the standard swaging process. The bolt is finished with a high polished nickel with Teflon. The extractor is highly modified and in fact grabs 20% more of the rim than the standard extractor. This spreads the forces of the extractor over a larger area decreasing the chance of the rim being ripped through or deformed causing malfunction. The “whale tail” extractor also uses two extractor springs with buffers inside of them. Both springs are pressers that decrease stress burdens on either spring. The buffers decrease “extractor bounce.” This is a condition where the extractor violently bounces off of the cartridge rim causing a failure to extract.

The Sharps Rifle Company Enhanced bolt “Relia-Bolt”

The steel insert carrier key that screws into the bolt carrier and is then pinned in place.

The Sharps Rifle Company entry took a different approach to bolt locking lug strength as well as reliability of the bolt to function inside of a dirty barrel extension. The Sharps entry was designed by Michael Blank. The bolt carrier remained unchanged being the standard chrome lined (inside and carrier key) manganese phosphate finish. The early versions used a proprietary carrier key that permitted the use of a cam pin with a round roller on the top instead of the squared off pin. The roller cam pin was first used by Gene Stoner in early AR-10 prototype rifles. The purpose is to reduce friction and drag of the bolt carrier inside the upper receiver. It also slightly decreases the damage the cam pin makes to the rear of the cam slot on the inside of the upper receiver. This rotary cam pin is more beneficial on an external piston gun but it has other benefits Blank was looking at.

The asymmetrical lug design was arrived at once they had better modeling software. It was thought that since the bolt only turns in one direction, there was the ability to further bias the curved surface in that direction, which is exactly what was done. This allowed for the maximum amount of curvature to help the bolt cam back into timing should it be out of sequence when it attempts to pass through the barrel extension. The tops of the lugs are still angled down so that there is not only a forward cutting edge still present for pushing through any debris in the barrel extension, but also it provides area where debris can collect in the forward part of the barrel extension without causing a jam. You still have to clean the action regularly; this isn’t supposed to replace that by any means but merely to increase the tolerance to debris and harsh climates and poor maintenance conditions.

The bolt was designed to be part of a system that made the M16/AR platform both stronger and more reliable. This was to include an improved bolt carrier with improved gas key, firing pin retaining pin, and the bolt with other improvements in the system including the barrel extension, which was able to achieve a sub 2 millionths of an inch surface roughness on spot inspection and a maximum peak valley surface roughness number of less than 10 millionths of an inch across a 20 inch barrel.

On top is the current production and available Relia-Bolt and on the bottom the original bolt that Blank submitted to the PIP program for testing.

The geometry of the bolt locking lugs was also changed. The lugs are rounded tapered lugs that push past carbon and debris when the square edges of traditional bolts would typically jam. The bolt compensates by providing a patented camming action that guides the bolt past obstructions and into battery. As designed the bolt is 100% compatible with a mil-spec bolt.

Although the Army canceled the bolt carrier PIP improvement for the AR claiming there were no significant advances, Sharps knew they had a viable product that did in fact outperform the standard bolt and decided to offer it to the commercial market as the Relia-Bolt. This is an updated version of the bolt that makes some changes. The bolt is machined from S7 tool steel that is much stronger than the standard mil-spec Carpenter 158. The S7 steel provides a 75% increase in tensile strength and nearly 60% improvement in yield strength. The bolts are tested with a 70,000 psi proof cartridge and then magnetic particle inspected to insure against stress fractures. Also, as per military specifications, the bolt is Shot Peened for added durability.

In 2013, Sharps Rifle released to the industry a further advanced Relia-Bolt. The geometry of the locking lugs has been altered as well as the bolt has been coated with an NP3electroless nickel-based finish. The process co-deposits sub-micron particles of Teflon for self-lubricating corrosion resistance. The final design is manufactured from S7 tool steel that is significantly stronger than mil-spec Carpenter 158 tool steel providing a 75% increase in tensile strength and nearly 60% improvement in yield strength. All bolts are tested with a 70,000 psi proof cartridge and then magnetic particle inspected to increase performance under pressure.

Bolt Carrier Group Finishes The search for the Ultimate Finish

Young Manufacturing chrome bolt after 500 rounds lubricated.

For quite some time, the industry has searched for a “lubeless” finish for the bolt carrier group. Based on this author’s experience, there is no such thing. Anytime two pieces of metal rub there is friction and with friction comes wear. With that being said, that does not mean that there are not finishes that hold up better to friction than others and will be less forgiving than another without lubrication. There is another issue that comes with the search for an ideal finish, one that is easy to clean and will not hold fouling and carbon. For the most part throughout U.S. military history, the standard Parkerizing was used. This finish was prone to come off leaving the material susceptible to corrosion. With the invention of the AR-10/AR-15, there was a different route taken. Stoner designed his bolt and carrier to be chrome plated making for a more reliable and easier to clean system.

Chrome Plating/Smith Enterprises and Young Manufacturing Chrome Bolt and Carrier

The first AR-15/M16 and XM16E1 rifles were issued with the Stoner designed chrome plated carrier groups – designed this way for several reasons. First, chrome is corrosion resistant. Second, was it has self-lubricating properties to it and thirdly it is easy to clean. Carbon and fouling will for the most part rub off with a cleaning rag without the use of solvents or oils. Back in the early 1960s this chroming process was expensive. Another issue was the hydrogen embrittlement issue, which was mostly due to improper manufacturing techniques. Modern manufactured chrome components do not have this issue since the process has been perfected and permitted by modern plating techniques. There were several variations of the appearance of the components from matte grey to shiny silver. This had to do with how the metal was polished
prior to plating.

The original early 1960s chrome bolt carriers are hard to come by. The bolt carriers are excellent and can be used with no issues. However, the early chrome bolts are not recommended. They were manufactured to original pre-ball powder specifications and will not hold up to the much higher stressed/higher cycle rate of modern ball propellants. They have not been strengthened or shot peened. Current very high quality chrome bolt carrier groups are manufactured by Smith Enterprises and Young Manufacturing, both of which this author has tested extensively in many conditions including heavy automatic fire. Young Manufacturing also offers a low mass carrier for those who use an adjustable gas system enabling you to balance the gas system and decrease recoil.

Micro Slick/Cerakote

Smith Enterprises manufactured chrome plated bolt carrier group. This is based on Gene Stoners original design who was a proponent of chrome plating.

Micro Slick is a dry-film lubricant designed to withstand extreme temperatures and pressures. The coating is designed to lubricate, wear resistance, and to act as an anti-seize agent while also preventing galvanic corrosion or the bonding of metals. Micro Slick is designed to be unaffected by temperatures ranging from below zero to 1,200 °F. This material is used in the engines of NASCAR race cars in areas such as piston skirts, cylinder walls, crank and camshafts, bearings and gears, valve stems and rocker pivots. To put this in perspective, a barrel on an M4 will achieve transformation temperature and rupture between 1,200 to 1,450°F. The bolt carrier group will never achieve this temperature; the barrel would rupture long before. When two pieces of metal are both coated with Micro Slick, the metal never touches metal. This in turn reduces dramatically friction and wear. In the AR-15/M16 rifle to achieve optimal benefit, both the inside of the upper receiver as well as the bolt carrier group should be coated with Micro Slick. The coating is a bluish grey. This finish is applied to the AR family of weapon’s bolts, bolt carrier, and trigger groups. Micro Slick is a ceramic coating that increases reliability, reduces galling, reduces heat and friction thus allowing consistent smooth movement of the bolt in the bolt carrier. There is a permanent bond with the metal that penetrates a few microns deep and is very thick (.25-.5 mils is the recommended thickness).

The rifle tested was manufactured by STS Arms and was the ST4 Improved MOE. STS claims to have done extensive testing that included 1,800 rounds on a dry bolt carrier group without cleaning. They then continued exceeding 3,000 rounds with the only malfunction caused by fouling plus time and air moisture changes (causing powder fouling to expand). The failures were only initial loading related. During testing of the STS rifle, the bolt carrier group was not lubricated and tested in the Nevada desert on a windy day. No malfunctions were experienced. The bolt carrier was removed with visible fouling on the inside and outside. The bolt still locked and unlocked freely. The outside was wiped down with a rag. There was still hard carbon on the back tail of the bolt that was easily removed with an Otis Tech bronze all-purpose brush.

NiB-X/WMD Guns

WMD Guns’ NiB-X is an extreme nickel boron coating. The coating has a hardness of 70 Rockwell and a Coefficient of Friction of .06. NiB-X can be applied to all ferrous and non-ferrous materials except magnesium. NiB-X provides a hard, lubricious surface to complex geometries. According to WMD, the advantages of NiB-X is that it is harder than Melonite (nitriding), nickel Teflon and hard chrome. NiB-X is environmentally corrosion resistant as well as lubricious. The finish has an extremely slippery feeling to it and is extremely easy to clean.

Microslick coated bolt carrier group. This is a dry film lubricant that works very well. To get the best results you would also coat the inside of the receiver.

“NiB-X stands not only for the coating itself but also for the process and expertise used to apply it to firearms. While the specifics of application and of obtaining the superior bond, look and feel is proprietary, suffice it to say that every aspect of it has been engineered specifically for firearms,” says Wynn Atterbury, President of WMD Guns. WMD Guns consists of a cadre of firearms and surface finishing experts that uses an exclusive high volume, high quality plating facility that is NADCAP accredited, the highest process control accreditation you can achieve in the Aerospace sector. These qualifications are important considering that there appear to be new entries in the nickel boron marketplace and not all nickel boron coatings are the same in quality of application or performance. There is even some confusion in the marketplace now between Nickel Boron and electroless nickel/boron nitride, a much softer and different coating. Nickel Boron properties and application are guided by and should be certified to AMS2433C and ASTM B607. NiB-X conforms to both of these specifications.

The NiB-X bolt carrier group was tested in a WMD Guns BEAST rifle. The entire bolt carrier group was coated in NiB-X except for the firing pin retainer pin. There were 400 rounds fired through this rifle without lubrication in the Nevada desert with no malfunctions of any sort. When the bolt carrier group was removed from the rifle there was visible fouling along with some visible sand.

The bolt carrier group was wiped clean with a rag. Everything within reach of the rag wiped right off. The only area that was difficult to clean was the rear of the bolt where the heavy carbon was. With the assistance of an Otis bronze all-purpose brush, this carbon came right off. The color of the bolt carrier group remained the same as it was brand new.

Nickel Boron/UCT

The original firearms application of nickel boron was pioneered by UCT or Ultra Chem Tec. One of the first real applications of this process was used by Colt Defense on their SCAR-Light entries. All three rifles were completely UCT coated. The UCT nickel boron can be applied to all ferrous and non-ferrous materials. The aluminum receivers, barrels as well as bolt carrier groups were coated. The fire control group was also coated. The rifle faired very well in the competition but would lose to the FN entry. We should note the SCAR-Light program was dropped due to SOCOM not getting what they wanted out of the rifle. It never left developmental stages before being cancelled. Further testing at Colt showed there to be corrosion issues and it was eventually dropped. This finish has been modified through the years and is now offered by Fail Zero.

Fail Zero/UCT bolt carrier group disassembled showing the fouling of 300 rounds with no lubricant.

The Fail Zero auto hammer and bolt carrier group were placed in an LMT Guardian rifle. There were 400 rounds of M855 ball ammunition fired through the rifle. Mostly the rifle was fired on automatic. Upon examination of the bolt carrier group there was obvious fouling. Also noted was the gas rings were worn out in just 400 rounds. This could have happened for any number of reasons but it appeared as though the gas rings were defective. They were replaced and the rifle fired again with no issues. The fouling appeared to be heavy and the bolt carrier group was disassembled. Heavy fouling was noted on the bolt. The carrier and bolt were wiped off with only a rag. Everything came off that the rag could come in contact with. When the bolt carrier was cleaned it was noted that the finish had a tarnished look to it. It was scrubbed again but the tarnish did not go away. The surface seemed to have the same slippery feel so it appeared as though this was only cosmetic and not a failure of the nickel boron.

Robar NP3 Nickel Boron with Teflon

Robar has been in the plating business for a number of years and the process they offer is called NP3. This is a “self-lubricating” nickel Teflon finish that is claimed to have self-lubricating properties, increased corrosion protected, increased surface hardness, increase the life of the parts as well as easier to clean. The NP3 finish is done for many OEM on their components.

The carrier was tested with 100 rounds of ammunition dry and no malfunctions were noted. The bolt carrier was virtually wiped clean with a rage. That is anywhere the rag could get to. The only real point of contingency was the back of the tail of the bolt that every finish test still had a build up there.

BLACKNITRIDE/Melonite by Rubber City Armory

The UCT/Fail Zero Nickel Boron coated bolt carrier group is one of the earliest advanced finishes to be used going back to the early 2000s. Colt had all three of their SCAR entries coated in this material.

The black nitride bolt carrier group used in testing was manufactured by Rubber City Armory. Every component was coated including the firing pin, extractor and firing pin retainer pin. BLACKNITRIDE or Ferritic Nitrocarburizing, is the thermochemical that simultaneously diffuses nitrogen and carbon into the surface of ferrous metals. During the process, a two-part surface layer is formed with an outer iron nitride layer with a nitrogen diffusion layer below it. During this treatment, nitrogen and carbon are absorbed by and diffused into the surface of the metal. Reproducible and uniform layers with a predetermined thickness will be formed on all areas of the metal. The result of this process is markedly improved surface properties on ferrous metal parts while maintaining dimensional and shape stability.

Metal components treated with BLACKNITRIDE have increased wear-resistance, fatigue strength, corrosion resistance, lubricity and cosmetic appeal. Often replacing the more expensive processes of hard chrome or nickel plating, BLACKNITRIDE gives better corrosion protection at equivalent or lower costs. Additionally, BLACKNITRIDE is commonly used as an alternative to high temperature conventional heat treatments such as carburizing and carbonitriding.

The Rubber City Armory Black Nitride bolt carrier group was tested in an LMT Guardian selective fire rifle. There were 400 rounds fired – mostly in automatic. The fouling was visible on all components and wiped off very easily with the rag – perhaps one of the most easies of all that was tested. Every area where the rag could reach, the fouling was removed. There is always a common thread as to all these carrier groups tested that hard carbon remains on the tail of the bolt. The only point of comparison is if it was easier to remove on any one finish compared to the lot. This author would have to say this was probably the easiest to clean. Keep in mind the carrier only sat dirty for a few hours and it was only 400 rounds.

Rubber City Armory has also released an adjustable gas key. The gas flow is controlled by a hex screw in the back of the carrier key. This enables the shooter to balance the gas system enabling the least amount of recoil necessary to operate the action resulting in better controllability on follow up shots as well as extending the service life of the components.


TR Enabling Obsidian Finish


The Robar finish is called NP3 and is on the Robar bolt carrier group. NP3 is a combination of nickel and Teflon.

Obsidian is a newer coating offering both increased performance and cosmetic appearance. According to TR Enabling, Obsidian has a coefficient of friction of .06. The plating process has an Obsidian thickness of.0005 inches and a Rockwell hardness of RC70. This graphite color finish is applied by an Electro-less plating process and the composition is a nickel based coating impregnated with Teflon. According to TR, enabling the guide rails of the carrier have been hand polished as well as the mouth of the gas key. The carrier finish is designed to function reliably on all heat and cold conditions. The finish is designed to resist carbon build up. The finish allows most carbon deposits and gun powder residues to be wiped off with a cloth making for easy field maintenance The Obsidian coating can be cleaned with any type of CLP or solvent.


The purpose of this article is not for the author to tell you what is the best finish for the bolt carrier group. The reality is all of these finishes are in fact superior to that of the standard GI manganese phosphate. You really cannot go wrong with any of them. Some such as the Black Nitride, Chrome, UCT and NP3 have quite a following in the OEM market. Like most things, your personal preference comes into play. Do you like a chrome color, black color or a nickel color? Of all the ones tested, the most time tested is in the old fashioned chrome. Chroming process are significantly better and now brought more to a science compared to the early days of Vietnam. You can certainly go on any firearms blog and see some very strong opinions. From my research, there is no magic finish that works lubeless. Anytime metal meets metal there is friction and there needs to be lubricants. These finishes tend to work a little better with just a small application.


This black nitride carrier group was fired 400 times completely dry. The fouling is visible.

Rubber City Armory bolt carrier group disassembled. Every component is finished in black nitride.