Weaver vs Picatinny Rails

Basically, they are similar in profile, but the recoil slots differ in width, depth, and spacing.

Picatinny Rail / Base: Slots are 5.23mm wide and 3mm deep, with a consistent spacing of 10.01mm.

Weaver Rail / Base: Slots are narrower at 4.57mm and tend to be shallower than the Picatinny standard. Slot spacing is variable depending on the manufacturer and model of rifle, and there tends to be only a small number of slots depending on whether it is a 2-piece or 1-piece base.

This means Weaver rings can fit both Weaver and Picatinny rails, whereas true Picatinny rings will only fit Picatinny rails.