Dazzle your friends with your knowledge of picatinny and weaver rails:

The "Picatinny Rail” is a term that has evolved in the firearm industry from a military standard, specifically MIL-STD-1913 (AR) which was adopted on February 3, 1995. The title of the publication is "Dimensioning Of Accessory Mounting Rail For Small Arms Weapons” and this document specified exactly what the dimensions and tolerances were for any mounting systems that were to be submitted for acceptance by the military. The term "Picatinny” comes from the place of origin for this system, the Picatinny Arsenal located in New Jersey. MIL-STD-1913 specifies the dimensions required for consideration, including length, width, height, and angles and the tolerances allowed for each measurement. The key distinction of the MIL-STD-1913 lies in the specification for the profile and the recoil groove.

What are the differences between the "Picatinny” and the "Weaver” systems? The profile of the two systems is virtually identical. Depending on the quality of the machining done by the manufacturer, the two systems should be indistinguishable from the profile. The key difference lies in the placement of the recoil grooves and with width of the grooves. MIL-STD-1913 (Picatinny) grooves are .206” wide and have a center-to-center width of .394”. The placement of these grooves has to be consistent in order for it to be a true "Picatinny” MIL-STD system. Weaver systems have a .180” width of recoil groove and are not necessarily consistent in a center-to-center measurement from one groove to the next. In many instances, a Weaver system has a specific application that it is machined for, so interchangeability is not necessarily an issue. A MIL-STD-1913 system must adhere to the specifications listed above in order for it to be considered MIL-STD, since the military desires uniformity in the recoil grooves to allow for different systems to be mounted on the weapon with no concern for compatibility.