[LexBaird]

True. But, as in the example of crying "fire" in a crowded theater, no restriction is acceptable before some harm has actually been committed.

No, intent to commit harm is also restrictable before the act. Carrying a gun is a right. Strapping on a gun with the announced intent of murdering someone is probable cause for arrest.

[TigersEye]

You're splitting hairs. Voicing a threat is committing an act of harm just as falsely crying "fire" in a crowded theater is. Possession of the means to fulfill the threat isn't even necessary to be arrested and convicted of a crime.