grabbed from wiki:
Quote:
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The only difference between the Picatinny rail and the Weaver rail is the size of these slots, although many rail-grabber-mounted accessories can be used on either type of rail. Weaver rails have a slot width of 0.180 in (4.572 mm), but are not necessarily consistent in the spacing of slot centers. The Picatinny locking slot width is 0.206 in (5.232 mm) and the spacing of slot centers is 0.396 in (10.058 mm).[1] Because of this, Weaver devices will fit on Picatinny rails, but Picatinny devices will not always fit on Weaver rails.
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ETA: since weaver devices fit pic rails, but not always the other way around it sounds to me like you have a pic optic and a weaver rail.