Something to look into might be a high-speed linear CCD or CMOS array; they do make 2D versions (which are used in high-speed digital video cameras; that is, those kinds of video cameras that can "shoot" many thousands of frames per second); two of those (one each for X and Y positioning) oriented at right angles to each other, along with IR illumination -might- be fast enough to "catch" the bullet as it passes. Whether such sensors exist, how easy they are to interface with, and whether the Arduino is fast enough to clock the data out - all completely unknown...