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For example:This is no joke.This is not a joke. Is the former more forceful?Is that the only difference?
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"No joke" is understatement or figurative. A case of encephalitis is no joke. You could not say that it is not a joke. "Not a joke" is literal and is not what you call an expression or anything.

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To me as a native speaker, they are the same. If written, the context will determine the force. If spoken, the situation and body language used will determine the force.