What You Need To Know About The Difference Between The Weaver, Picatinny, And NATO Specifications

Understanding the difference between the: Weaver, Picatinny, and Nato Specifications

By now we have all seen some sort of specification referenced when looking at Optic mount/adapters. The question is what do they mean and how does it affect compatibility?

Weaver base

First, I find it helpful to shed a little background on the subject.  William Ralph Weaver created the first successful mounting base.  There were other companies that made mounting bases, but Weaver’s version gained the most notoriety. Prior to the Weaver base, Gunsmiths would “custom” mount scope rings to receivers. This usually required drilling/tapping of the receiver to accept the screws that attached the scope rings to the receiver.

Picatinny rail 

The Weaver base is still a popular option on today’s firearms. It is the foundation for the Picatinny rail system that was created by the US Military in 1995. Whereas the earlier Weaver design is based on a low wide dovetail, the Picatinny variation has a more pronounced angular section. This allowed for the mounting of heavier optics and accessories, due to more surface area for “clamping.”

NATO accessory rail

The most recent revision (2009) to the popular Picatinny rail standard (know as MIL-STD 1913 or STANAG 2324) is the NATO accessory rail. The NATO accessory rail standard (known as STANAG 4694) builds upon the platform of the Picatinny rail system. In fact, there are only a few very slight modifications to the Picatinny rail system.
According to the NATO Army Armaments Group the differences between the MIL-STD 1913 Picatinny rail and the STANAG 4694 are:

  • A metric reference drawing.
  • Additional new measurements and tolerances.
  • Adjustments of some measurements.
  • Reduction of straightness tolerances with approximately 50%.

The most notable difference, in my opinion, is the clamping surface engagement. With the Picatinny rail system, the V-angles are used for the alignment and reference of the accessory (clamping engagement shown in RED).  Whereas the NATO accessory rail utilizes the top surface for the alignment and reference of the accessory.

It is surprising to us that, according to NATO testing, the Picatinny rail system does not provide good repeatability. According to their (NATO) tests, utilizing the top surface as a reference and alignment of the grabbers provided excellent repeatability. NATO testing defines “repeatability” as the ability to maintain the same alignment of the accessory when it is removed and then reinstalled on the accessory rail.



Let’s first dive into the NATO and Picatinny Specs, since they are so close. For all intents and purposes, Picatinny and NATO accessories will work across both platforms. Meaning, a Picatinny accessory will work on a NATO rail and a NATO accessory will work on a Picatinny rail.


Now let’s throw the Weaver base into the mix.  A general rule of thumb is:  Weaver accessories fit on NATO and Picatinny rails.  However, Picatinny and NATO accessories will not always fit onto a Weaver Base.  Let’s clarify a little further what portions of the Weaver accessory are not compatible.

First, the width of the slots is less on a Weaver base when compared to the Picatinny/NATO bases.  This affects accessories that have an incorporated recoil lug (see below image).  The recoil lug on the Picatinny/NATO accessory will not always fit into the slots of a Weaver base, due to its size.

Second, the spacing from slot to slot is consistent on a Picatinny/NATO accessory rail.  On the Weaver base, the spacing is inconsistent.  Therefore, a Picatinny/NATO accessory that has multiple recoil lugs is going to rely on the consistent pattern that the Picatinny/NATO utilizes.  This will not work with the Weaver base because it does not possess consistent slot to slot spacing.

Examples of Picatinny Rail – recoil lug – spacing


The below accessory chart summarizes the compatibility between the Weaver/Picatinny/NATO rails/bases and their respective accessories.

Outerimpact rails/bases are compliant with both the Picatinny and NATO standards.  You can be confident that when you purchase a rail/base from Outerimpact that you are receiving a product that adheres to the tight tolerances provided by the Picatinny/NATO standards.

Accessory – referring to all possible items that can be mounted on a specific platform (i.e. telescopic sights, tactical lights, laser aiming modules, night vision devices, reflex sights, foregrips, bipods, etc)

NATO – North Atlantic Treaty Organisation

STANAG- a Standardization Agreement (STANAG) defines processes, procedures, terms, and conditions for common military or technical procedures or equipment between the member countries of the alliance. Each NATO state ratifies a STANAG and implements it within their own military.

MIL-STD – A United States defense standard, often called a military standard, “MIL-STD”, “MIL-SPEC”, or (informally) “MilSpecs”, is used to help achieve standardization objectives by the U.S. Department of Defense.

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