The Gaussian beam shooting (GBS) technique is well suited for problems of high frequency scattering by complex geometries thanks to the spatial and spectral limited extents of paraxial Gaussian beams (GBs). In this work, we focus on the GBS-based analysis of monostatic scattering from open-ended cavities comprising piecewise planar surfaces. Such geometries require relatively simple implementation while facilitating study of the algorithm’s accuracy and stability when GBs undergo multiple reflections. In particular for cavities, the number of reflections depends on the angle of incidence, thus exhibiting non-trivial accuracy and complexity behavior.