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The Picatinny rail (// or //), also known as a MIL-STD-1913 rail, Standardization Agreement 2324 rail, or tactical rail, is a bracket on some firearms that provides a standard mounting platform consisting of rails with multiple transverse slots similar in concept to to the earlier commercial Weaver rail mount used to mount telescopic sights.
Whereas the earlier Weaver design is based on a low wide dovetail the Picatinny variation has a more pronounced angular section. Designed to mount heavy sights of various kinds a great variety of accessories and attachments are now available and the rails are no longer confined to the rear upper surface (receiver) of long arms but are either fitted to or machine milled into the upper, side and lower surfaces of all manner of weapons from crossbows via pistols and long arms up to an including anti-materiel rifles.
Accessories - which can include vertical 'tool' or 'pistol' grips; bipods and rests; electro-optical sights including image intensifiers; flashlights and laser sights - can be mounted by sliding into place (or if already fitted with a Weaver mount clamped to the rail), thus providing backward (one-way) compatibility with any items already provided with Weaver-type mounts.
As mentioned the rail consists of a precision crafted strip undercut to form a flattened T cross-section provided with crosswise slots at intervals interspersed with flats that allow accessories to be slid into place from the end of the rail then locked in place; slid into the slots between raised flats then moved a shot distance back or forth or clamped to the rail with bolts, thumbscrews or levers (as per the Weaver system).
The Picatinny locking slot width is 0.206 in (5.23 mm). The spacing of slot centers is 0.394 in (10.01 mm) and the slot depth is 0.118 in (3.00 mm).
The only significant difference between the Picatinny rail and the similar Weaver rail are the size of these slots and the fact that they are standardized.
(Weaver rails have a slot width of 0.180 in (4.57 mm), but are not necessarily consistent in the spacing of slot centers. As already mentioned while some accessories are designed to fit on both Weaver and Picatinny rails most Picatinny devices will not fit on Weaver rails but from May 2012 most mounting rails are cut to Picatinny standards and as many shooting accessories can be fitted with just a single recoil pin this obviates issues of incompatible slot spacing.)
The rail itself dates from work by the A.R.M.S. company in the early 1980s and Otto Repa in standardizing the Weaver design. Specifications for the M16A2E4 rifle and the M4E1 carbine received type classification generic in December 1994. These were the M16A2 and the M4 modified with new upper receivers where rails replaced hand guards.
The rail is named after the Picatinny Arsenal in New Jersey, USA. The Picatinny Arsenal's role with the rail was to test/evaluate it and to create a military standard for it. This was Mil-STD-1913, dated February 3, 1995.
The Picatinny rail was originally for mounting scopes atop the receivers of larger caliber rifles, but, once established, its use expanded to other such accessories as tactical lights, laser aiming modules, night vision devices, reflex sights, foregrips, bipods, and bayonets. Because of their many uses, Picatinny rails and accessories have replaced iron sights in the design of many firearms, and they are also on the undersides of semi-automatic pistol frames and grips. Their usefulness has led to their being used in paintball and airsoft.
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