The testing considered (1) the ability of the average police officer to deliver effective fire, (2) the relative effectiveness of hits at different locations and depths of penetration into a human target, (3) rapid incapacitation as the preferred effect (independent of eventual death), (4) the performance of bullets in a reproducible target medium (ordnance gelatin), and (5) a method to extend the ranking system to variations in cartridge loadings without an extensive new test program. The kinetic energy theory of Hatcher and the energy deposit theory of the U.S. Army and Dr. DeMaio were examined and found to be lacking, and a new measure of handgun bullet effectiveness against human targets was developed. The new measure, called the Relative Incapacitation Index (RII), is the product of the volume of the Maximum Temporary Cavity produced by the interaction of a projectile and the tissue simulant (gelatin) at a given depth and the average vulnerability to incapacitation at that depth summed together for the entire penetration depth of the bullet to a maximum depth of 22 cm. The study concludes that there is no ideal firearm system (weapon/ammunition) for all situations; each police department must evaluate its own special requirements and choose a defensive weapon system capable of meeting its needs; however, the study shows that for handguns in 9 mm/38-caliber to 45-caliber range, a deforming projectile, driven at a velocity above the minimum deformation velocity, and an RII between 10 and 30 is a reasonable goal for handgun ammunition for use against a normally dressed assailant in an urban environment. Appendixes contain references, tables of coefficients for the RII predictive equation, the Theoretical Cavity Model for nondeforming projectiles, and vulnerability studies. For the second volume, which presents experimental data, see NCJ 93840.