M-LOK, for Modular Lock, is a firearm rail interface system developed and patented by Magpul Industries. The license is free-of-charge, but subject to an approval process.

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Quick Facts Type, Place of origin ...

M-LOK
Type	Attachment System
Place of origin	United States
Production history
Designer	Magpul Industries
Designed	2007–2014
Produced	2014–present

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M-LOK allows for direct accessory attachment onto the "negative space" (hollow slot) mounting points, and is a competing standard to VLTOR's open sourced KeyMod system for replacing the ubiquitous Picatinny rail in some applications.^[1] Compared to the Picatinny rail system, both M-LOK and KeyMod enable the user to have a slimmer, lighter, smoother and more fenestrated handguard/fore-end with accessories & gadgets mounted only where needed, Whereas a Picatinny handguard typically will have whole length rail slots, resulting in heavier weight, bulkier handguard with sharp edges not user friendly, and poorer barrel ventilation increase the speed of overheating.

The M-LOK system can be seen as an evolution of the Magpul Original Equipment (MOE) system, but the two are not fully compatible. Though newer M-LOK accessories can be used on older MOE slot handguards if an adaptor plate is used, there is no adaptor available for using older MOE accessories on the newer M-LOK handguards.

A prototype of the MOE slot was revealed by Magpul in late 2007 together with their Masada Concept Rifle (which would later be known as the Adaptive Combat Rifle). Magpul released the MOE slot system in 2008 as a feature on their MOE handguards, and at the same time compatible accessories such as Picatinny rail sections, direct MOE mounted light mounts, grips, bipod studs, etc. were released.

The MOE slot standard was never officially released, and a drawback to the system was that the rear side of the panel had to be accessed in order to mount accessories, limiting its application. The MOE slot system uses a weld nut which has to be placed manually on the inside of the handguard before mounting, making the slot system unsuited for applications such as free-floating handguards^{[citation needed]}. Also, depending on the accessory item, the spacing increments between the MOE slots were not small or uniform enough to adjust the desired placement of accessories.

Acknowledging shortcomings of the MOE systems, Magpul drafted M-LOK as a new and improved mounting standard which was released in 2014 replacing the existing MOE slot. The M-LOK rail specification included metric dimensions instead of imperial, and utilizes a T-slot nut capable of only 90-degree rotation, reinforced by thread-locking fluid, making it suited for applications on free-floating handguards. It was designed to work with both metal and polymer parts.

In 2016, Colt Canada developed and released the Modular Rail Rifle (MRR) that uses a monolithic upper receiver with the M-LOK attachment system.^[2] In 2017, several companies produce M-LOK handguards as well as accessories like Picatinny rail strips, vertical foregrips, bipods, sling adaptors, and flashlight mounts.^{[3][failed verification]}

In 2017, a summary report of testing conducted by NSWC-Crane for USSOCOM indicated that, while comparable in endurance and rough handling testing, M-LOK greatly outperformed Keymod in repeatability, drop testing and failure load testing.^[4]

In 2018, the Upper Receiver Group-Improved (URG-I) is a U.S. Army Special Operations Command (USASOC) program to further improve the durability and reliability of the SOPMOD Block II by introducing additional component improvements. First fielded in the same year, the main improvements are the lighter Geissele Mark 16 free-float rail that incorporates M-LOK as the mounting method and a Daniel Defense cold hammer-forged barrel that returns to the lighter "government" profile contour as well as a mid-length gas system. Used on the M4A1 carbines by ASOC then MK18 URG-I variant CQBRs by AFSOC.

While M-LOK is licensed free-of-charge, it is not open source, and thus manufacturers must acquire a license from Magpul before making products using the M-LOK standard. Magpul claims this gives them more control in assuring that all M-LOK products are made to specifications ensuring compatibility. Program participation is open to any interested manufacturer.^[5]

Note that although Magpul describes the license as a "free license", the meaning does not match the more common meaning originally established in computer circles. The computer "free license" refers to a freedom to use,^[6] while Magpul only conditionally offers the license for free (analogous to a source-available freeware).

NATO standards conform military issued assault rifles like the CAR 816 have semi-automatic only export versions featuring M-LOK rails on the handguard to better meet civilian market preferences in the United States.^[7][8][9]

• KeyMod—competing standard open standard design to M-LOK for mounting accessories
• NATO Accessory Rail—further development from the MIL-STD-1913
• Picatinny rail (MIL-STD-1913)—improved and standardized version of the Weaver mount. Used for both for scope mounts, and for accessories (such as extra sling mounts, vertical grips, bipods etc.) Major popularity in the civilian market.
• Rail Integration System—generic term for a system for attaching accessories to small firearms
• SOPMOD
• UIT rail—an older standard used for mounting slings particularly on competition firearms
• Weaver rail mount—early system used for scope mounts, still has some popularity in the civilian market
• Zeiss rail—a ringless scope mounting standard