The category of arms which represent the group of Personal Defense Hand-held Weapons (PDHW) has special characteristics and requirements; among this wide range of characteristics is the request to gain high power of fire while retaining high mobility incorporated with high accuracy of fire. The old design 9mm Parabellum which is still inservice
has lost the credibility of securing the demands of the 20th century soldier; this deficiency is represented in its low muzzle energy in spite of high recoil force which causes projectile instability during flight and loss of firing accuracy. In addition its bullet possesses bad ballistic shape which decreases the effective range. In this paper, a comparative study was performed between two pistol calibers, the 9mm old Parabellum and the same arm when modified and substituted by 5.56x23mm caliber. The exterior ballistics of both calibers was carried out using the PRODAS package. The aerodynamic coefficients and the trajectories were predicted and compared for both projectiles. The study reveals that using 5.56 mm caliber improves the power of fire, as well as the accuracy and stability. The study assures the importance of substitution of the in service 9mm Parabellum with the 5.5623mm which satisfies all combat and economical requirements as a personal defense hand hold weapon.