Picatinny rail

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M4A1 Modular Weapons System carbine with M1913 rails on upper receiver and foregrip

Picatinny Rail Dimensions, Cross Section (dimensions in inches)

The Picatinny rail (IPA: /ˈpɪkətɪni/ or /ˌpɪkəˈtɪni/) is a bracket used on some firearms in order to provide a standardized mounting platform for telescopic sights and other accessories, such as tactical lights and laser sighting modules. The standard was first published by the Picatinny Arsenal and carries the official title MIL-STD-1913. It is also known by the NATO designation STANAG 2324.

The rail is typically placed directly on the firearm's receiver, in the position normally occupied by the rear sights. The rail's cross-section is shaped roughly like a wide T. Scopes are mounted on the rail either by sliding them on from one end or the other, or by means of a "rail-grabber" which is clamped to the rail by means of bolts, thumbscrews or levers.

In order to provide a stable platform, the rail should not flex as the barrel heats and cools. For this reason Picatinny rails are cut crosswise, to give them considerable room to expand and contract lengthwise. These slots are also standardized in size, and many accessories use them for locking purposes. The Picatinny locking slot width is 0.206" (5.2324mm). The spacing of slot centers is 0.394" (10.0076mm) and the slot depth is 0.118" (2.9972mm).^[1] The only difference between the Picatinny rail and the similar Weaver rail is the size of these slots, although many rail-grabber-mounted accessories can be used on either type of rail. Weaver rails have a slot width of .180", but are not necessarily consistent in the spacing of slot centers.^[2] Because of this, Weaver devices will fit on Picatinny rails, but Picatinny devices will not always fit on Weaver rails.^[3]

Originally used for telescopic sights, the rails were typically found only on larger caliber rifles. With the increased use of night vision devices, they started to appear on smaller assault rifles as well, to the point today where they are displacing the original iron sights of many firearms with versions that can be mounted on the rails.

Once the rails became more common, they started to be used for other accessories, such as flashlights. This has led to many accessories now designed rail-mounted, including bipods, bayonets and laser sights. In turn, this had led to the introduction of "small rails" that can be fastened to guns in various locations, and in some cases, entire grips with rails built-in on all sides. Short rails now appear even on shotguns and pistols.