A modular dovetail rail clamping assembly includes a body having a fixed clamp component. A movable clamp component opposes the fixed clamp. A threaded clamping fastener extends through the movable clamp component and into the body to draw the movable clamp component into engagement with the body. A removable, reversible crossbar extends transversely across the body between the fixed clamp component and the movable clamp component in a spline channel. The crossbar has both a universal rail side and a MIL-STD 1913 rail side.