are weaver and picatinny rings/mounts compatible?

The profile of the two systems is virtually identical, depending on the quality of the machining done by the manufacturer,

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”. 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.