A rail integration system (RIS; also called a rail accessory system (RAS), rail interface system, rail system, mount, base, gun rail, or simply a rail) is a generic term for any standardized attachment system for mounting firearm accessories via bar-like straight brackets (i.e. "rails") often with regularly spaced slots. Rail systems are usually made of strips of metal or polymer screw-fastened onto the gun's receiver, handguard, or fore-end stock to allow variable-position attachments. An advantage of the multiple rail slots is the moveable positions to adjust for optimal placement of each item for a user's preferences, along with the ability to switch different items at different placements due to varying eye reliefs on gun sights. Firearm accessories commonly compatible with or intended for rail systems include tactical lights, laser sights, vertical forward grips, telescopic sights, holographic sights, reflex sights, backup iron sights, bipods/tripods, slings, and bayonets. The common types of rail systems for firearms are the dovetail rail (including the Soviet variant known as the Warsaw Pact rail), the Weaver rail, the Picatinny rail, the SOPMOD, the KeyMod and the M-LOK. There are also various non-military designs used in shooting sports to attach slings and bipods such as the UIT rail, Zeiss rail and Freeland rail.
History Original rails were a raised metal strip with the sides undercut, less standardized than the dovetail design, to allow hardware to slide on and be secured by means of compression only.
Design
Rail systems are usually based on the handguard of a weapon and/or the upper receiver. Modern pistols usually have rail systems on the underside of the barrel. Rails on rifles usually start at the top dead center ("12 o'clock"), with other common placements at the bottom 180° ("6 o'clock") and on the sides at 90° ("3 o'clock" and "9 o'clock"); some rails are also diagonal at 45° angles as opposed to 90° angles, though these are less common. There may be additional attachment rails or holes at each 45° angle position running partially or entirely the length of the handguard.
On the Kalashnikov rifles, the Warsaw rail is attached to the left side of the receiver when viewed from the rear. With more modern versions adding Picatinny style rails onto the sides of the handguards of the rifles for the mounting of additional equipment. Due to updating equipment, both styles may be found on some Warsaw Pact weapons. Modern-designed firearms often include rails made into the body, instead of being an added-on modification. Older firearms may need permanent modifications of having holes drilled and tapped for screw threads to fasten the rail sections to the firearm. This is easier than milling out a dove tail slot for the placement of a gun sight's parts. Optics such as telescopic sights, reflector sights, holographic sights, red dot magnifiers, night vision sights, or thermal sights may be placed between the iron sights. The rail section may also come in various heights to help align equipment, which may align with the original iron sights inline or below an illuminated optic's center dot, ring or chevron. This is referred to as absolute or lower 1/3 co-witness respectively. In addition to height variations some rail brackets may be offset at various degrees. 22.5°, 45°, and 90° are the most common, to place accessories and/or backup folding collapsible iron sights in such a way so that they are out of the line sight on the top of the firearm and/or to decrease the outer profile edge's size. Then, the original sights are a backup if the electronic optic should fail. The rail section may also move weapon-mounted lights forward so the light does not shine and reflect on the firearm directly, which may create shadows. The amount of rail space allows adjustment and personal optimization of each device and tool attached for the user. As designs have advanced the amount of space has succeeded in the actual need for placement space. Thus, rail covers and protectors may be added to prevent snagging on gear and/or plant foliage. Future rails systems may have the option of carrying batteries or other electricity systems to supply the needs of the increasing electronics mounted to aid the shooter. Standards are still being determined for these types of systems. An example of such is NATO standards NATO Accessory Rail which is the continued improvement and standardization of the Picatinny rail.
Types
Most RIS equipment is compatible with one or more of the most common rail systems, all of which are broadly similar:
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