Rail Interface System are the primary method of attaching accessories to small firearm

Firearm

A firearm is a weapon that launches one, or many, projectile at high velocity through confined burning of a propellant. This subsonic burning process is technically known as deflagration, as opposed to supersonic combustion known as a detonation. In older firearms, the propellant was typically...

s such as assault rifle

Assault rifle

An assault rifle is a selective fire rifle that uses an intermediate cartridge and a detachable magazine. Assault rifles are the standard infantry weapons in most modern armies...

s and light machine gun

Light machine gun

A light machine gun is a machine gun designed to be employed by an individual soldier, with or without an assistant, as an infantry support weapon. Light machine guns are often used as squad automatic weapons.-Characteristics:...

s. 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

Picatinny rail

The Picatinny rail or MIL-STD-1913 rail or STANAG 2324 rail or a "Tactical Rail" is a bracket used on some firearms in order to provide a standardized mounting platform. A similar system is the Weaver rail mount.-Description:...

 standard, which was the first standard developed.

There are different styles of RIS, depending on how much the operator will be accessorizing the weapon. Basic systems that can be installed on weapons such as the previously mentioned AR-15/M16 are usually small rails (20mm standard) with holes machined in them that are screwed onto the existing hand guard on the rifle and can cost as little as $25 to $40 USD

United States dollar

The United States dollar , also referred to as the American dollar, is the official currency of the United States of America. It is divided into 100 smaller units called cents or pennies....

. More advanced systems allow for numerous accessories to be mounted simultaneously and cost an average minimum of $200 USD and up for a decent quality system.

Common accessories include tactical light

Tactical light

A tactical light is a flashlight used in conjunction with a firearm to aid low light target identification, allowing the marksman to simultaneously aim and illuminate the target. Tactical lights can be handheld or mounted to the weapon with the light beam parallel to the bore...

s, laser aiming modules, forward hand grips

Foregrip

A foregrip is the grip on the front of a firearm that helps to control the gun, and prevent burns from the barrel during firing. A vertical foregrip can also allow for easier horizontal movement of the firearm, as it creates a firmer grasping point than conventional hand guards. Some firearms...

 for improving weapon handling, telescopic sight

Telescopic sight

A telescopic sight, commonly called a scope, is a sighting device that is based on an optical refracting telescope. They are equipped with some form of graphic image pattern mounted in an optically appropriate position in their optical system to give an accurate aiming point...

s for medium ranged or distant targets, and reflex sights/red-dot sights for short to medium ranged targets, iron sight

Iron sight

Iron sights are a system of shaped alignment markers used as a sighting device to assist in the aiming of a device such as a firearm, crossbow, or telescope, and exclude the use of optics as in telescopic sights or reflector sights...

 lines, bipod

Bipod

A bipod is a support device that is similar to a tripod or monopod, but with two legs. It provides significant stability along two axes of motion .-Firearms:...

s, and bayonet

Bayonet

A bayonet is a knife, dagger, sword, or spike-shaped weapon designed to fit in, on, over or underneath the muzzle of a rifle, musket or similar weapon, effectively turning the gun into a spear...

s.