The paper considers fragmentation process of high-explosive projectile casings, i.e. rapidly expanding cylinders loaded by extreme internal pressure generated by detonation of explosives. The classical, physically based, Mott model of the ring fragmentation is examined and the adequate computer program is realized. The fragment size (or mass) distribution is analyzed and the average fragment size is related to the characteristics of expansion and the casing material properties. The influence of the fragmentation process parameters on the nature of fragment length distribution is analyzed. The theoretical distributions are compared with experimental data and good correspondence is obtained.