The best caliber is taken to be that one which yields the greatest effective range under constraints on weapon recoil energy and system weight. For bullets and guns having a particular configuration, relationships are developed which express these constraints. Semi-empirical expressions for recoil energy and system weight are obtained as functions of caliber and muzzle velocity. When energy and weight are constrained, a feasible region is defined in the two-dimensional space of caliber and muzzle velocity. Within this feasible regions, the greatest effective range is found at the smallest caliber (and largest muzzle velocity.) (Author).