High Quality Content by WIKIPEDIA articles! Rail Interface System (RIS, sometimes also referred to as Rail Accessory System, RAS) are the primary method of attaching accessories to small firearms such as assault rifles and light machine guns. This is done primarily in militaries and by firearm enthusiasts to improve the usability of the weapon being accessorized quickly and efficiently without requiring the weapon operator to field-strip the weapon. Most quality RIS systems are built to the M1913 Picatinny standard, which was the first standard developed.