Nucl.Instrum.Meth. A531 (2004) 467-480 A Shashlyk calorimeter prototype for the KOPIO experiment has been constructed and experimentally tested. The energy resolution of about 4%/sqrt(E(GeV)) for 0.5-2.0 GeV/c positrons was obtained. Based on this results as well as on the results of special measurements, a Monte-Carlo model of the Shashlyk module response was developed. This model, including the effects of shower evolution, light collection in scintillator plates, light attenuation in fibers, quantum efficiency of the photodetector, thresholds and noises in the readout system is consistent with experimental results. Possible improvment of the Shashlyk energy resolution up to 3%/sqrt(E(GeV)), the level required by KOPIO experiment, are discussed.