Here:Originally Posted by TeeJay
http://en.wikipedia.org/wiki/KeyMod
KeyMod is an open-source design released for use and distribution in the public domain in an effort to standardize universal attachment systems in the firearm accessories market. The KeyMod system consists of two parts. The KeyMod slot, and the KeyMod nut. The slot is distinctive with a larger diameter through-hole combined with a narrow slot. The slot is chamfered on the backside while the through hole is sized for clearance of a quick-detach sling swivel (approximately 3/8" diameter).
The nut is stepped and the larger diameter end is chamfered around 270 degrees of its diameter. The angled face created is meant to interface with the chamfer on the backside of the KeyMod slot. The full diameter is left intact to create two flats on the nut which align the nut to the slot, and allow it to be indexed to the accessory as well as to the KeyMod slot. This eliminates the need to align the nuts to the holes prior to accessory installation as well as the need for tools to tighten the screw/nut. In most accessories, the screw is swaged after assembly to ensure that it cannot be backed out of the nut. This prevents loss of small parts (screws, nuts or other small parts used in the assembly of the accessory). The spacing of the holes is critical and is based on MIL-STD-1913 spacing to allow the greatest modularity with existing accessories.
The KeyMod specifications call out a "recoil lug" on the accessories which is intended to interface with the larger through hole portion and resist slippage of accessories during counter-recoil. The combination of the angled interface of the nut to the KeyMod slot and the recoil lug to the through hole make for a very strong attachment point which will not slip under harsh recoil or counter recoil. It also provides for an excellent return-to-zero when removed and re-installed.
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