C-7 foaming explosive
From Halopedia, the Halo wiki
C-7 foaming explosive is a highly explosive compound that is used in demolitions by UNSC forces. It is comprised of various chemicals that react to a signal electrical current shock sent through a detonation switch. It is liquid in vacuum, but once contact with a hard surface and oxygen occurs, it becomes a foaming, semi-solid adhesive resin.
C-4 is short for "Composition 4", therefore it is likely that C-7 means "Composition 7." It is also likely that it is simply a codename based on the order in which it was developed, not an indicator of its explosive power.
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