Shashlyk electromagnetic calorimeter modules with an energy resolution of about 3%/sqrt{E (GeV)} for 50-1000 MeV photons has been developed, and a prototype tested. Details of these improved modules, including mechanical construction, selection of wa
ve shifting fibers and photo-detectors, and development of a new scintillator with improved optical and mechanical properties are described. How the modules will perform in a large calorimeter was determined from prototype measurements. The experimentally determined characteristics of the calorimeter prototype show energy resolution of sigma_E/E=(1.96+-0.1)% oplus (2.74+-0.05)%/sqrt{E}, time resolution of sigma_T = (72+-4)/sqrt{E} oplus (14+-2)/E (ps), where photon energy E is given in GeV units and oplus means a quadratic summation. A punch-through inefficiency of photon detection was measured to be epsilon = 5*10^{-5} (Theta >5 mrad).
