A new numerical procedure by which field calculations in AMOS are upgraded to model rotationally symmetric cavity structures in a more accurate fashion is described. The development work is aimed at implementing a modified finite difference update scheme on an irregular grid system. Elements of an irregular grid may be chosen to better fit object boundaries, resulting in increased solution accuracy. Our approach involves the placement of field components on a non-orthogonal body fitting grid and on a dual grid which is orthogonal to the first grid. It is found that this procedure retains several important computational advantages, including the ability to exploit the implied spatial relationships between nodes. Propagating fields on an irregular grid system have been observed and comparisons between finite difference AMOS and Modified-Yee AMOS field calculations are provided.