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In the late 1990’s, US SOCCOM had experienced significant issues with failures to extract as well as bolt structural failures on their M4A1 rifles. The problem was serious enough where Karl Lewis was approached to design an improved bolt carrier group that would correct the shortcomings of the standard GI bolt carrier group. The new design would improve the durability and reliability of the M4A1 carbine for SOCOM. LMT delivered their Enhanced Bolt Carrier Group because failure was not an option.

The Bolt Carrier had several modifications. The most significant being the modification to the cam track to allow more dwell time before the bolt would unlock giving the cartridge case more time to contract, making extraction easier. Due to the shorter gas system on the carbine the bolt unlocks and extracts sooner than on a standard length rifle. This “too soon” action causes the bolt to unlock while the cartridge case is still slightly expanded making the extractor work harder to pull the cartridge case from the chamber. Sometimes the extractor may slip off causing a failure. The modified cam track allows more time allowing the cartridge case to contract. The gas exhaust port was moved up and forward on the carrier. This permits more gas to be vented out of the ejection port, rather than inside of the receiver. This results in a cleaner rifle.

The change in the location also regulates the peak pressure to slow the action down. 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. Additionally, the gas port in the bolt carrier is divided into two channels instead of one large port. These two ports slow down the gas flow and direct the pressure to each side of the bolt rather than on top of it.

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The Bolt is highly modified to deal with several common issues found within the heavy firing schedules used by SOCOM. First, the bolt is made of a much stronger material. This material is more costly, as well as the cost of the tools to cut them. The bolt lugs are stress relieved by the radius’s cut into each lug. The lugs are allowed to flex. The LMT bolt is the first bolt which fully supports the base of the cartridge case adding strength. The “Lobster Tail” extractor is a dramatic improvement over the standard with a more aggressive extractor claw that engages more of the rim. This enables a better grip on the rim to aid in extraction under harsh conditions. Instead of using one extractor spring, the enhanced bolt uses two extractor springs. This gives a significant improvement in the life of the extractor springs, as well as extractor force. 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 ensure against stress fractures in the material. The locking lugs have rounded edges instead of squared off edges.

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Comments

Hoosier Ed
01/18/2018 3:05am

Chris: Thanks for doing this article. Ever since you talked about the "Enhanced Bolt Carrier" in one of your YouTube videos, I wanted more detailed information on this product. The LMT website does not provide the information I was looking for. Not that I have a full auto AR, but didn't you say in one of your videos this BCG was not compatible with full auto ARs due to the dwell time changes?

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Chris Indermuehle
01/18/2018 2:14pm

Chris,

Did they do a statistical study after LMT made these changes to support tjat these changes had the desired results? If so, do you have these numbers?

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    Technical Specialist, 10 years in forensics, veteran, technical writer and armorer instructor.