A model is developed to describe the evolution of a solid armature to the point where metallic contact with the rails is lost. The idea is that a current/melt wave begins at the rear of the armature, where current is concentrated by velocity skin effect, and propagates forward along the rail/armature interface as molten armature material is lost. When the melt wave reaches the front of the armature, the transition to plasma brush contact occurs. A calculation with the model simulating an earlier solid armature experiment at General Atomics shows close agreement with the measured transition velocity, as inferred from the increase in muzzle voltage observed during the shot.