An improved clamping device for a weapon accessory rail of a type having an elongate mounting structure of generally T-shaped cross-section. The rail mounting structure may be of a type including a mounting surface and first and second inclined proximal surfaces on opposite sides of the mounting surface and first and second inclined distal surfaces adjacent the first and second proximal surfaces, respectively. The clamping device includes a mounting shoe with a first inward surface for engaging the mounting surface and a second inward surface for engaging the second distal surface. The mounting shoe has an outward facing surface opposite the first inward facing surface for attaching an accessory device. A cam lever pivotally movable between first and second positions has a rotary cam surface engaging a bearing member. The bearing member is movable between a clamped position and unclamped position in response to movement of the cam lever to the first and second positions, respectively. In operation, the bearing member exerts a clamping force on the first distal surface of the mounting structure in a direction orthogonal to the first distal surface.