I think it's a good ruling. But I would consider the smarts of the antagonist. If it is not very bright, the silence may be targeted at the user of the horn and not the horn itself. I would probably assign a chance-in-six, based on the antagonist's IN, WS, and experience. Even the brightest sorcerer with a lot of experience might, on a 1-in-6 chance, target the person and not the object.
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