Can you crack open the moon without disturbing the natural magnetic balance? No. Are weapon designs able to change over the next couple centuries? Yes. What would be more bothersome?
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The laws of physics mean that we will never develop a weapon in which the projectile fired from it is not aligned with the barrel at the time of firing. Ultimately, anyone who would actually design such a thing likely wouldn't be around for very long, so I can truthfully say the whole moon thing would be more bothersome, yes.