Two of the most popular mounting rails for scopes, and now many other accessories, are the Weaver and the MIL-STD 1913 Picatinny, usually called just "Picatinny". They look similar but they ARE different. Brownells Firearm Support Technician Caleb Savant tells us the nitty gritty on HOW they differ, starting with their origins. (Hint: think William Weaver of Weaver scope rings and the Picatinny Arsenal in New Jersey.) The Picatinny has a precisely defined set of exact dimensions and tolerances, while the Weaver is a bit more loosey-goosey. An accessory designed to fit a true Picatinny rail will not fit a Weaver rail, but most Weaver accessories will fit a Picatinny rail. BUT for the most secure installation, it's always best to match a particular ring to its corresponding base.
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