This is quite widely known fact - in early 80x USSR tested captured tank with m111 round against T-72A. Only glacis were penetrated from less than 1000m range (3 from 5 hits). As result of this trial 20mm steel appliqué was added on glacis to all T-72A in russian service. T-72B / T64B / T80 had better protection, so obviously they were safe.