What is the Difference Between Weaver and Picatinny Bases?

The difference between Weaver and Picatinny Base

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

Picatinny Rail / Base slots are 5.23mm wide and 3mm in depth with a consistent spacing of 10.01mm.

Weaver slots are narrower at 4.57mm and tend to be shallower than picatinny standard.  Also the slot spacing is variable depending on the manufacturer and model rifle they are made to fit, also there tends to be only a small number of slots depending on a 2 piece or 1 piece base.

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