I doubt anyone can say "Avada Kedavra" faster than a speeding bullet. Also, bombs. And tanks. And drones. And armies.
English
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For one, good wizards don't need to speak the spells. Secondly, wizards can control technology.
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Have you seen the later films? Spells are cast without the words.
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Only a few wizards are that powerful. It would be easy to kill them with snipers, helicopters, or anything else. In fact, are there even any spells in the series powerful enough to destroy any sort of vehicle?
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I would guess there are explosive spells but I can't remember