Product Code Database
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.
M-LOK allows for direct accessory attachment onto the "negative space" (hollow slot) mounting points, and is a competing standard to VLTOR's KeyMod system for replacing the ubiquitous Picatinny rail in some applications. KeyMod vs. M-Lok: The Next AR Rail Standard by Chris Baker, November, 19, 2014 Both M-LOK and KeyMod enable the user to have a slimmer, lighter, smoother and more fenestrated handguard/fore-end with accessories and gadgets mounted only where needed as compared to a Picatinny handguard, which typically has whole length rail slots, resulting in a heavier weight, bulkier handguard and poorer Gun barrel ventilation, resulting in the barrel overheating more quickly.
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.
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. 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 system dimensions instead of imperial units, 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.
and M-LOK handguards from various manufacturers]]In 2017, several companies produce M-LOK handguards as well as accessories like Picatinny rail strips, vertical foregrips, , sling adaptors, and tactical light mounts. M-LOK Outselling KeyMod about 3 to 1, 28 January 2016
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.
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.
Note that although Magpul describes the license as a "free license", the meaning does not match the free license. The computer "free license" refers to a freedom to use, Open Definition 2.1 on opendefinition.org "This essential meaning matches that of “open” with respect to software as in the Open Source Definition and is synonymous with “free” or “libre” as in the Free Software Definition and Definition of Free Cultural Works." 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.
Attachment screws made by many U.S. manufacturers are often either #8-32 TPI or 10-24 TPI UNC threads, which respectively have major thread diameters of 0.1640 inches and 0.1900 inches (4.166-0.794 mm and 4.826-1.058 mm expressed in metric designation). Many M-LOK screws on the international market instead use either M4 or M5 metric threads to reduce cost.
The tool required for mounting, in addition to the thread size, also depends the screw head type. Hex keys are used extensively in the firearms industry, but metric and imperial hex keys as a general rule are not compatible. It is possible to damage the tool and screw by selecting a tool that is too small for the fastener, which can be done by using an imperial tool on a metric fastener, or the converse. An exception to this is 4 mm hex keys, which are almost the exact same size as . In many industries, this makes hex keys preferred for consumer products because end users can successfully use an imperial key on a metric fastener, and vice versa.
| M4 | 3 mm | 2.5 mm |
| M5 | 4 mm | 3 mm |
| #8-32 | " (3.57 mm) | " (2.38 mm) |
| #10-24 | " (3.97 mm) | " (3.18 mm) |
While screw and slot dimensions are available on the web, the T-slot nut dimensions are currently under review by the US State Department to determine whether it should be regulated by ITAR, and until it is clarified drawings are only available to US citizens.
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