We quantify the discrepancy between radar cross section (RCS) of long cylindrical scatterers and RCS of infinite structure with the same cross-section. The scatterers are made of perfect electric conductor and are excited so that vector of electric field is perpendicular to the cylinder axis. We use method of moments (MoM) for numerical analysis. The results show that if the cylinder is longer than 10 wavelengths, the 3-D RCS can be predicted with mean squared discrepancy less than 1 dB using 2-D analysis.