Rail system (firearms)

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A dovetail rail on a rifle receiver for mounting a sight (i.e. a scope or iron sights).
Romanian PSL rifle with Warsaw Rail a variation of a dovetail used for scopes from 1954. Becoming common on the AK- ### series as of 1992
The M4 carbine with a Picatinny rail system on the upper receiver and 4 sided handguard, showing a few possible accessory options such as a Grip Pod vertical forward grip, and an M68 CCO sight

Rail systems are small strips of metal or polymer put on or removed from any given surface on a gun to allow attachment of some sort of accessory. On them, accessories such as weapon-mounted lights, laser sights, scopes a Vertical or Angled grip and R.I.S systems can be attached. An example of just a few options that may be used depending on a mission's need for the M-4 is SOPMOD . Amongst many other accessories like sling attachment points. Which may be ether ring,loop and/or quick detach mounts using push button style hardware. To name just a few of the many options. An advantage with the multiple rail slots is the moveable positions to adjust for optimal placement of each item for each different user. Along with the ability to switch different items at different placements due to Eye Relief distances on gun sights, scopes & optics. Tactical usage and Shooting sports have both benefited from the extra options provided.

Rails as a term has evolved to cover both the actual rail piece(s) and the styles of hand guard(s) (or forearms) that are made with rails as the external surfaces. Original rails were a raised metal strip with the sides under cut, less standardized than the dovetail design, to allow hardware to slide on and be secured by means of compression only.[1] The firearm often associated with, and that has benefited from rails is the M4 Carbine & M16 family of later models/variants -A2 -A3 & -A4 . With equal and even more use due to additional novelty items is the AR-15. Most modern military and civilian semi-automatic Firearm have rails that may replace original parts. Police and Military style firearms may include Pistols, PDWs, Carbines, Rifles, Submachine guns, Light Machine Guns, and Heavy machine guns . HMGs have started to include and use rail sections and options for attachments of optics. Civilian clone Rifles are the largest adapters, while the Crossbows,[2] Hunting Rifles, shot guns and Handguns have started to come from the factory with rail sections ether attached and/or made structurally as part of the actual Firearm. Airsoft and Paintball clone weapons will also likely have rails.

Rail Designs[edit]

The seven common types of rail systems for firearms are the Dovetail rail,[3][4] Weaver rail,[5]Warsaw Pact rail, Picatinny rail aka MIL-STD-1913 rail,[6][7] KeyMod[8][9][8][10] and M-LOK[11][12][13][14] UIT Rail

MIL-STD-1913 "Picatinny rails" date from the mid-1990s and have very strict dimension and tolerance standards. The Picatinny has rail of very similar profile to the Weaver, but the slot width is 0.206 in (5.23 mm), and by contrast with the Weaver, the spacing of slot centers is consistent, at 0.394 in (10.01 mm). Many rail-grabber-mounted accessories can be used on either type of rail, and accessories designed for a Weaver system will generally fit Picatinny rails - although not vice versa. The Picatinny locking slot width is 0.206 in (5.232 mm) and the spacing of slot centers is 0.394 in (10.008 mm).[15] Because of this, with devices that use only one locking slot, Weaver devices will fit on Picatinny rails, but Picatinny devices will not always fit on Weaver rails.[15]


KeyMod and M-LOK handguards from various manufacturers. The tops of these handguards feature Picatinny rail.
A combination hand guard with Picatinny rail on top with M-LOK slots on the side with 2 QD/ Flush cups and, a 5 slot Picatinny rail section accessory showing the mounting hardware

Adapters to other types of rail interfaces maybe used for legacy issues and/or to change the surface texture, abrasiveness and/or overall outer circumference of the entire rails system for fit of the hand. Dovetail rail, Weaver and Picatinny are all outward or raised attachment surfaces. While M-Lok and KeyMod have smooth surfaces with different standards & styles of holes cut into their assembles to place the attachment hardware internally. Both features are often on and/or in the hand guards . All make the mounting and dismounting of these objects significantly easier. Items may be fasten by thread bolts, requiring use of a screwdriver or allen wrench, with some tool free variations of a Thumb screw(s) or Thumb nut(S), may have threaded quick disconnect lever that pulls the hardware and plates together against the rails. During firearm recoil the accessory may slide within that section of rail. To help avoid this, when tightening slide the device forward in the placement slots so the section of bolt is against the vertical/forward section of rail slot(s).


A U.S. Army Ranger, armed with a M249 light machine gun, Showing multiple Rails and Accessories in overwatch security on an objective during a mission in Iraq, 2006.

Rail systems usually are based on the handguard of a weapon and/or the Upper receiver. On modern pistols they are on the underside of the barrel. Rails on rifles usually start off at top dead center or 12 o'clock with the next placement at bottom 180° degrees or at 6 o'clock. on the forward section, away from the buttstock. With both sides 3 o'clock & 9 'o clock or 90° degrees each side of top center 0°/360° degrees becoming the 3rd & 4th most common. 1 o'clock or 11'o'clock is popular for the flashlight with atape switch location placed on user preference. There may be additional attachment rails or holes at each 45 °degree angle position running partially or entirely the length of the handguard.

On the Kalashnikov rifles AK style/family 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 on to the sides of the handguards of the rifles for the mounting of addition equipment.

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 section(s) to the firearm. This is easier than milling out a dove tail slot for placement of a gun sights parts.

Optics like scopes, Reflex Sight, and Red Dot may be placed between the iron sights. 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°, & 90° are the most common, to place accessories and/or back up folding collapsible iron sights. So 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 back up if the electronic optic should fail. The rail section may also move a weapon-mounted lights forward so the flashlight(UK torch) does not shine and reflect back on the firearm directly. Creating shadows and visual impairment from this illumination.

The amount of rail space allows adjustment and personal optimization of each device and tool attached for the user. As designs have advance the amount of space has succeeded the actual need of placement space. Thus rail covers and protectors may be added, to prevent snagging on gear and/or plant foliage.

The six common types of rail systems for firearms are the Dovetail rail, Weaver rail, Warsaw Pact rail, Picatinny rail, KeyMod and M-LOK.

Future rails systems will have the option of carry power to supply the needs of the increasing electronics mounted to aid the shooter and/or soldier. Standards are still being determined for multiple countries. An example of such is NATO standards NATO Accessory Rail which is continued improvement and standardization of the Picatinny rail.

See also[edit]

Dovetail rail

Weaver rail

Warsaw Pact rail

Picatinny rail



UTI rail

Rail Integration System(RIS)

NATO Accessory Rail which is backwards-compatible with the STANAG 2324 or MIL-STD 1913 Picatinny rail.


  1. ^ Muramatsu, Kevin (14 July 2014). Gun Digest Guide to Maintaining & Accessorizing Firearms. Krause Publications. pp. 168–169. ISBN 978-1-4402-3989-2.
  2. ^ Todd A. Kuhn (13 December 2013). Shooter's Bible Guide to Bowhunting. Skyhorse Publishing Company, Incorporated. p. 289. ISBN 978-1-62873-404-1.
  3. ^ Picatinny, Weaver & dovetail scope bases
  4. ^ Muramatsu, Kevin (16 June 2014). Gun Digest Guide to Maintaining & Accessorizing Firearms. Krause Publications. pp. 169–. ISBN 978-1-4402-3989-2.
  5. ^ Inouye, Kevin (30 May 2014). The Theatrical Firearms Handbook. Taylor & Francis. pp. 364–. ISBN 978-1-317-85981-9.
  6. ^ Sweeny, Patrick (30 August 2016). The Bench Manual Gunsmithing the AR-15. Gun Digest Books. pp. 284–. ISBN 978-1440246609.
  7. ^ Rros and Cons Picatinny Rail
  8. ^ a b Slowik, Max. "New Open-Source KeyMod Universal Mounting System by Vltor". Guns.com. Retrieved 3 August 2015.
  9. ^ Curtis, Rob. "VLTOR gives the keymod tdp to you". Militarytimes.com. Archived from the original on 2014-02-26. Retrieved 2013-04-09.
  10. ^ MLOK vs KeyMod 2016
  11. ^ Magpul Industries - M-LOK DESCRIPTION AND FAQ DOCUMENT
  12. ^ KeyMod vs. M-Lok: The Next AR Rail Standard by Chris Baker, November, 19, 2014
  13. ^ KeyMod vs. M-LOK Modular Rail System Comparison, Presented by Caleb McGee, Naval Special Warfare Center Crane Division, 4 May 2017 full pdf on page
  14. ^ M-LOK vs KeyMod comparison 2017
  15. ^ a b "Picatinny Rails, Weaver Rails, What’s The Difference?" Brownells