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10x20mm 1.04 ton Femtonuke

Currently the smallest nuclear weapon I'm aware of, weighs 3.79 grams, costs about a third of a credit, and has a yield of 1.04 tons of TNT. Essentially a harmless X-ray flashbulb for annoying your enemies, only small enough to fire out of a handgun, and much, much less dangerous than it would be if the game modelled radiation correctly.
(Also physically impossible if the game modelled fission correctly)

Requires modded material Octaazacubane.