US Signal, Practice Bomb, Mk 4 Mod 3, 4
Practice bomb signals Mk 4 Mods 3 and 4 are essentially 10-gauge shotgun shells. They contain an expelling charge of smokeless powder and are primed with a commercial primer. A pyrotechnic or inert marker load is separated from the expelling charge by a disc and cardboard gun wad. The end of the shell is closed by felt gun wads which are cemented to the cover.
Signal Mk 4 Mod 0 was the first of this type developed. Mods 1 and 2 were procured later for issue to activities limited by environment to performing practice bombing in the vicinity of flammable areas. These signals contain inert materials which produce very little flash and are markedly inferior to the Mod 0. Signal Mk 4 Mod 3 is similar to the Mk 4 Mod 0 but differs in that the cartridge case of the Mk 4 Mod 3 is extruded aluminum instead of paper; a primer mixture with improved storage characteristics has been used, and a new pyrotechnic load (which produces about the same flash, but with a superior smoke puff) has been incorporated. The Mk 4 Mod 4 signal is similar to the Mk 4 Mod 3 with the exception of an inert marker load of zinc oxide. In both Mods, the cover and cartridge case are cemented together; in Mod 3 the assembly also is staked in four equally-spaced places.
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