from
brownels web site
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.

Now, what does this mean to you? Boiled down, it means that accessories designed for a Weaver system will, in most cases, fit on a “Picatinny” system. The reverse, however, is probably not the case. Due to the larger recoil groove, “Picatinny” accessories will not fit a Weaver system. There are, of course, exceptions to every rule, but for a good rule-of-thumb, “Picatinny” won’t fit Weaver, but Weaver will fit “Picatinny”.