"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%.
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."
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