A 3D nonlinear FEL formulation based upon a Coaxial Hybrid Iron (CHI) wiggler is described. The CHI wiggler is created by insertion of a central rod and an outer ring (composed of alternating ferrite and nonferrite spacers) along the axis of a solenoidal. Relatively high field strengths can be achieved with the CHI wiggler at shorter wiggler periods than is possible in many other conventional wiggler designs. The formulation is a slow-time-scale analysis of the interaction of an annular electron beam with the CHI wiggler in a coaxial waveguide. Simulations are presented for W-band operation. The results indicate that operation over a wide bandwidth is practical, and that the uniform- and tapered-wiggler bandwidths are comparable.