Practice bomb signal cartridges are used for day and night operations and as a visual indication of bomb impact and fuze function. Three types of signal cartridges are discussed in this section. Mk 4 Mod 3 signal cartridge. The Mk 4 Mod 3 practice bomb signal cartridge (fig. 4-11) consists of an aluminum case. The rim end has a percussion primer, and the opposite end is sealed with an aluminum disk. The cartridge produces a flash of light and a white smoke puff. The Mk 4Mod 3 is primarily used for night operations. CXU-3A/B signal cartridge. The CXU-3A/B practice bomb signal cartridge (fig. 4-12) consists of a cylindrical aluminum case. The case is closed on one end with a percussion primer, and the opposite end is closed with a crimped-in-place aluminum cover (cap). The cartridge produces a dense, white smoke display for day missions. CXU-4/B spotting charge. The CXU-4/B spotting charge (fig. 4-13) is a fire-free signal used with practice bomb BDU 45/B to provide a daytime visual indication of bomb impact point and fuse function.