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TypeAttachment System
Place of originUnited States
Production history
DesignerMagpul Industries
Magpul MOE handguard on a user-assembled AR-15 semi-automatic rifle

M-LOK, standing for Modular Lock, is a free licensed[1] firearm rail interface system patented by Magpul Industries, developed by the Magpul design team consisting Michael T. Mayberry, William Bradley Bennett, Timothy Eric Roberts, Duane Liptak and Brian L. Nakayama.

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.[2] 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 mounted only where needed, whereas a Picatinny handguards typically will have rail slots for its whole length resulting in a heavier and bulkier handguard with sharp edges and poorer barrel ventilation.

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). The MOE slot system was released by Magpul in 2009 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 an 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 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.[3] In 2017, several companies produce M-LOK handguards as well as accessories like Picatinny rail strips, vertical foregrips, bipods, sling adaptors, and flashlight mounts.[4]


While M-LOK is free licensed, 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.

Technical specifications[edit]

Rail specifications[edit]

The slot dimensions (used on handguards, etc.) are available on the web.[5] The slots provide metric 20 mm (0.79 in) length intervals, and accessories can be mounted either within a slot or bridging between slots, making it possible to adjust the position of accessories in smaller intervals than the length of the slot. The slots on an M-Lok handguard are approximately 32 mm (1.260 in) long and 7 mm (0.276 in) wide and space 8 mm (0.315 in) from each other. The radius of the corners is approximately 2.38 mm (0.094 in).[6][7][8]

Attachment specifications[edit]

The quarter-turn T-slot nut attachment screws have Unified Thread Standard #8-32 threads (0.15625 inches (3.97 millimetres) major thread diameter. The torque specifications are:

  • For attaching metal accessories to metal handguards: 35 lb⋅in (4.0 N⋅m)
  • For attaching polymer or metal accessories to polymer handguards: 15 lb⋅in (1.7 N⋅m)
  • For attaching polymer accessories to metal handguards: 15 lb⋅in (1.7 N⋅m)

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.

See also[edit]

  • Rail Integration System, generic term for a system for attaching accessories to small firearms
  • Weaver rail mount, early system used for scope mounts, still has some popularity in the civilian market
  • Picatinny rail (MIL-STD-1913), improved and standardzied 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.
  • NATO Accessory Rail- further development from the MIL-STD-1913
  • UIT rail, an older standard used for mounting slings particularly on competition firearms
  • KeyMod - competing standard open standard design to M-LOK for mounting accessories