The Mode Matching Technique is employed for the evaluation of the Radar Cross-Section (RCS) of cylindrical inlets terminated by a perfectly conducting cylindrical hub or a cylindrical array of grooves that may be straight or curved. In the case of curved grooves, the geometry is analyzed via the generalized transmission line theory and a closed form solution is given for the first time. The method is formally exact, and yields accurate RCS patterns for geometries that have not been previously investigated. Analytical expressions are derived for the coupling factors between the various modes and RCS results are presented which may be used as a reference in validating other, more general numerical techniques.