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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.
For this section, you can assume that Picatinny rail and NATO Accessory Rail are practically the same. In this section, we use "Picatinny rail" to refer to both rails. Read about their minor differences in the "Specification Differences" section .
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 following photo shows three difference size screw in the Weaver rail slots. The screw size are #10, #8, #6. Only the #6 screw fits the Weaver rail slot. The other two are too big. On the other hand, all three screws fits the Picatinny rail slots, even though the #10 is slightly pushing the limit.
The photo below shows the #10 screw leaving marks on the Picatinny rail. The #10 is just slightly too big, even though it works fine with the Picatinny rails.
When you look at the cross section of the Weaver rail and the Picatinny rail (see photos below), you'll see that the Weaver rail has a lower profile. The Picatinny rail is slightly raised, like a mushroom on its stalk.
There are some Picatinny-Weaver hybrid rails out there. The following photos shows one such hybrid. The top rail is a Weaver rail. The middle rail is the hybrid. The bottom rail is a Picatinny rail. The hybrid's slot width is the same as the Picatinny slot width. But the slot distance vary, like a Weaver rail. For this particular hybrid, I would still consider it to be a "Weaver" rail.
The following photo shows another hybrid on top of a scope ring. The rail on top is a Picatinny rail. The slot width are the same, but the slot distance are, against, different from Picatinny specs. I would also consider this hybrid a "Weaver" rail.
From these practical examples, you can easily see that accessories made for Weaver rails will work just fine on the Picatinny rail. Accessories for the Picatinny rail may work on the Weaver rail if they have these characteristics:
There is another variation to the Picatinny and Weaver rails. That is there is a perpendicular wide slot all the way down the rail as shown in the photo below.
The following photos, once again, makes the difference really clear. The Picatinny rail on the left does not have this perpendicular slot. The Picatinny rail on the right does have the perpendicular slot.
There is no real practical use for that slot, but it doesn't get in the way of using a Picatinny or a Weaver rail. I suspect that slot is to reduce cost in material.
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.