The propagation of electromagnetic wave in gun tubes, radiated by interior ballistic measurement radar, is analyzed according to electromagnetic scatter theory and waveguide theory. With the method of numerical computation of MOM, the relationship between the corresponding RCS and the wave frequency is figured out, and the effect of multi-mode on the SNR is discussed. Finally several conclusions are brought forward on the selection of working frequency for the radar. The conclusions are further verified by the results of a couple of live tests.