Is it permissible to throw enemy heads using a catapult (manjaniq)?

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It is disliked to throw heads using a catapult, a position explicitly stated by Ahmad ibn Hanbal. However, if such an action is done for a necessary public interest (maslaha), it is permissible. This exception is evidenced by the incident where Amr ibn Al-'As, during the siege of Alexandria, ordered a Muslim captive's head to be cut off and thrown via catapult to the enemy after they had thrown the head of a Muslim captive to the Muslims, in order to equalize the situation.