Ever wondered what the difference between a Weaver and a Picatinny rail is? This article from Brownells explains all.
Essentially because the Picatinny Rail is covered by a Mil-Std they should all be the same otherwise they are not a Picatinny rail. Weaver rails however, can vary in size depending on the manufacturer.
The Figures below are the dimensions of a Picatinny rail.