M-LOK
Also: MLOK
An open-source slotted attachment standard for fastening accessories directly to a forend or chassis, lighter than a full rail.
M-LOK is a negative-space mounting standard: rows of slots are cut straight into the forend or chassis, and accessories lock in with T-shaped lugs that draw up tight against the back of the slot. It saves weight and bulk over bolting a full Picatinny rail along the whole forearm.
On precision rifles, M-LOK shows up on chassis systems and modern stocks as the way to hang the gear that matters downrange, chiefly a bipod, plus arca adapters, sling points, and weights. You add a short rail section or a direct-attach mount only where you actually need one.