Weaver and Picatinny Rails
So you have a new scope that you want to mount on your rifle. The scope came with a set of rings that are Weaver style. All you need now is a scope base and you will be set to mount the scope on your favorite firearm. When you start looking for scope bases you will find two styles that look the same in the pictures, but are not the same. One is called Weaver and the other Picatinny (MIL-STD-1913). These two rails, in many cases, can be used interchangeably.
The main differences between the Weaver and the Picatinny rails are the size of the cross slots and the slot spacing. Weaver rails have a slot width of 0.180" (4.572 mm), but are not necessarily consistent in the spacing of slot centers. The Picatinny rail has a slot width of 0.206" (5.232 mm) and the spacing of slot centers is always 0.394" (10.008 mm). Because of this, Weaver devices will fit on Picatinny rails, but Picatinny devices will not always fit on Weaver rails.
So those Weaver style rings that came with your new scope will work on both styles of rails. If the scope has been supplied with Picatinny style rings, you will most likely be limited to only the Picatinny mount. Picatinny mounts and rings will most commonly be found on products that were originally designed for military use and have found their way into the civilian market. For instance, the top rail on an AR-15 (the civilian version of the U.S. M16 battle rifle) flattop receiver is a Picatinny rail. You are able to use both styles of rings on this rifle.