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.
Magpul MOE M-LOK handguard on a user-assembled AR-15 semi-automatic rifle
Colt Canada MRR with M-LOK rail handguard
The URG-I variant M4A1 carbine has M-LOK rail handguard with an LA-5C/PEQ on the top rail used by one of the Green Berets from 3rd SFG (A) during training at Marine Corps Air Ground Combat Center, Twentynine Palms, California in 2019
M110A1 SDMR featuring a Geissele M-LOK rail handguard
A rail integration system is a generic term for a standardized system for attaching accessories to firearms. Rail systems are straight mounting brackets on the gun's receiver, handguard, or fore-end stock to allow sliding or variable-position attachments. An advantage of the multiple rail slots is the moveable positions to adjust for optimal placement of each item for a user's preferences, along with the ability to switch different items at different placements due to varying eye reliefs on gun sights.
A rail system mounted on top of a SIG SG 550
A dovetail rail on a rifle receiver for mounting a sight
A Romanian PSL rifle with a Warsaw rail
A French Army soldier armed with an Heckler & Koch HK416 with multiple rail-mounted attachments, including a scope, a tactical light, and an M320 underbarrel grenade launcher
