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We present a characterization of contact velocity skin effect (VSEC), which is a major velocity- and efficiency-limiting effect at a railgun's sliding contact. Despite enormous contact forces, the armature remains separated from the rail by a thin layer, 4 to 12 A thick. Evidence suggests VSEC is also the primary mechanism responsible for the contact voltage drop. VSEC effects are seen in both electromagnetic launcher (EML) efficiency and breech voltage. We compare theoretical predictions of system efficiency and breech voltage to experimental measurements for both a conventional and an augmented railgun. The characterization of VSEC extends our previous theoretical work in this area and provides new insights into the physics of EML operation, especially with regards to the armature and sliding contact. VSEC is a significant energy loss mechanism and heat source, possibly contributing to contact erosion and transition. We propose a similar VSEC mechanism to explain velocity saturation and efficiency roll-off in plasma and hybrid armature railguns, as well as arc restrike.