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The significant difference between the Picatinny rail and the similar Weaver rail are the size and shapes of the slots, where the Picatinny rail has square-bottomed slots while Weaver rails have rounded slots. An accessory designed for a Weaver rail will fit onto a Picatinny rail whereas the opposite might not be possible. Some accessories are designed to fit on both Weaver and Picatinny rails, most Picatinny devices will not fit on Weaver rails.