The development of a predictive method for investigating the steady inviscid aerodynamic behavior of ballistic projectiles having various axisymmetric and nonaxisymmetric boattail shapes is reported. These shapes include the now standard conical boattail as well as a variety of nonaxisymmetr ic shapes. The theoretical procedure employs the classical transonic equivalence rule and a new transonically corrected apparent mass loading method. Theoretical results for surface pressures, loadings, and static aerodynamic characteristics are presented throughout the transonic range for a variety of projectiles having different boattail geometries. Comparisons with results of both experiment and other theoretical methods demonstrate the accuracy of the procedure. YPICAL projectiles in current use by the Army are slender, spin-stabilized bodies of revolution. The boattail configuration that has become the standard is a conical shape with a relatively shallow cone angle ~(5-»10 deg). The primary purpose of any boattail is to increase the projectile range by reducing drag from what it would be if the projectile afterbody were cylindrical. While drag reduction is ac- complished, an associated detrimental result is the creation of negative lift on the boattail. This tends to increase further the destabilizing pitching moment produced by positive lift on the nose, and thereby to reduce additionally the gyroscopic stability of the projectile. At transonic flight speeds, which usually occur near ballistic trajectory apex, the negative loading on the boattail is strongly augmented by the development and movement of shock waves on the boattail. This results in a rapid peaking of the destabilizing pitching moment at flight Mach numbers just below one. That and related changes of other aerodynamic characteristics can result in the projectile becoming unstable. In an effort to reduce the adverse transonic behavior of ballistic projectiles, the Army has recently investigated ex- perimentally1 a series of nonaxisymmetr ic boattail shapes. Some of these were found to improve significantly the aerodynamic characteristics over those of the conical con- figuration. In particular, it was found that increased gyroscopic and dynamic stability and decreased drag could be attained simultaneously. These findings are of considerable importance because they showed for the first time that projectiles designed with such shapes would have both in- creased range and improved stability compared with projectiles employing the standard boattail. The present work describes the development of a theoretical method for predicting the transonic static aerodynamic characteristics of these projectiles. The theoretical analysis for determining the nonlinear three-dimensional projectile flowfields is based on the classical transonic equivalence rule (TER). A new loading calculation method based on apparent mass concepts and which makes use of nonlinear TER flow solutions is used to predict the static aerodynamic coef- ficients. Theoretical results for surface pressures, loadings, and static aerodynamic coefficients are presented for a variety of projectiles with different boattail geometries at Mach numbers throughout the transonic range. Comparisons are made insofar as possible with both other theoretical methods and experimental results.