Using potential flow theory and the boundary conditions imposed by the bullet motion, the velocities impacted to the blood by the bullet impact were determined. The breakup of the blood upon impact was then described in the framework of the Rayleigh-Taylor instability, which occurs when a dense fluid (blood) breaks into drops when accelerated into a lighter fluid (air). Results of the model indicate that the size of the drops produced depends on their distance from the bullet path and on the bullet velocity and shape. Smaller, faster blood drops originate close to the impacting blunt bullet, and the larger and slower ones are formed further from it. Calculation of the drop trajectories is explained in this report. The discussion of the implications of this work for criminal justice notes that although this work is the first physical description of the production of a back spatter pattern due to gunshot, the implications are not immediate for criminal investigations. This is because the described project was a forward study in which results were obtained from known initial conditions; however, this does not create a model for reconstruction, in which initial conditions are determined from an inspection of the final conditions, i.e., the observed spatter. Two recommendations for crime-scene documentation are offered. Plans are underway for testing a reconstruction method with more spatters. 5 figures, 1 table, and 25 references