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.
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, [full-width] ‘Picatinny’ won’t fit Weaver, but Weaver will fit ‘Picatinny’.”