The general rule of thumb is that weaver accessories will attach to mil-spec picatinny rails... but picatinny accessories will not attach to weaver rails.
Picatinny slots are much wider (and deeper?) than weaver slots... so if you have mil-spec scope rings they won't fit into a weaver slot because the crossbar will be far thicker than the weaver slot can accomodate.
Hope that answers it for ya.