C-12 shaped charge
From Halopedia, the Halo wiki
C-12 shaped charges are used by the United Nations Space Command for heavy demolitions, typically as a military explosive. The C-12 compound is extremely dangerous and very volatile. A small 4x4x4 inch cube alone can level a five story building in seconds. It is solid, but is malleable enough that it can be shaped or bent into different forms to stick to surfaces better and fit into tight spots and is detonated with electric shock detonator sticks or remote detonators.
As C-4 is short for "Composition 4", it is probable that C-12 means "Composition 12". It is likely that "C-12" is a codename based on the order in which it was developed, not an indicator of its explosive power. In reality, C-2 has more explosive power than C-4, but it is also more unstable, and thus newer compositions were designed for better stability and malleability.
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