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There are currently three, loosely compatible, standard rail system that you will find on firearms, airsoft guns, and paintball markers. All three of them are closely related to each other. In this article we are going to provide the differences in a number of ways--layman term, practical examples, and specifications--to help you take advantage of the rail interface system (RIS).
Slot - On the rail, there are rectangular cuts (see photo below). They are called slots. The slots allow screws to run through them, which helps aligning accessories on the rails.
The following photo shows a Weaver rail (top) and a Picatinny rail (bottom). They both serve the same purpose, but have subtle differences. Both rails have the same width with the same angled features along the side edge. On both rail, the slot width is consistent. On the Picatinny rail, the slot distance from one another is consistent. But on the Weaver rail, the slot distance may vary. In fact, on a Weaver rail you may have any number of slots.
The Weaver rail has a width of 21.2mm. The Weaver rail may have slots. The slot have a fixed width of 0.180" (4.57mm). Other than that, the dimension of the Weaver rail is rather loose.
Picatinny rails are specified using MIL-STD-1913, and is later rectified as STANAG 2324, shown below. Note that almost every dimension is specified for this rail. All of the dimensions are specified in inches.
NATO Accessory Rail is specified using STANAG 4694, shown below. The specification is basically the same as the Picatinny rails. The biggest difference is the dimensions are specified in metrics system, rather than English system. Only other difference is that the NATO Accessory Rail allows a rounded base.
One addition specification difference between the Picatinny rail and the NATO Accessory Rail is at the Rail/Grabber Interface (see difference below). This difference is significant at the mechanical design level, not at the consumer level.