Rail Interface System (RIS, sometimes also referred to as Rail Accessory System, RAS) is a generic term for a system for attaching accessories to small firearms such as pistols, rifles and light machine guns.
Common accessories include tactical lights, laser aiming modules, forward hand grips for improving weapon handling, telescopic sights for medium-ranged or distant targets, and reflex sights/red-dot sights for short to medium-ranged targets, iron sight lines, bipods, and bayonets.
Most RIS equipment is compatible with one or more of the most common rail systems, all of which are broadly similar:Weaver rail mount - an early system, still popular in the civilian market
Picatinny rail (MIL-STD-1913) - standardized US military version
NATO Accessory Rail - developed from MIL-STD-1913
Keymod - open standard design to replace MIL-STD-1913 for mounting accessories (except for scope mounts)
M-LOK, a free licensed competing standard to KeyMod
These are used primarily in the military and by firearm enthusiasts to improve the usability of the weapon, being accessorized quickly and efficiently without requiring the operator to field-strip the weapon. Basic systems such as small rails (20mm is standard) with holes machined in them to be screwed onto the existing hand-guard of a rifle and can cost as little as US$25 to US$40. More advanced systems allow for numerous accessories to be mounted simultaneously and can cost upwards of US$200.