From the Super Mario Wiki
Magnum is a special attack used by Smithy in his tank head form during the final battle of Super Mario RPG: Legend of the Seven Stars. This attack is similar to Blazer, as both can instantly defeat a single party member and both have the same animation. When using this attack, Smithy will fire a single golden Bullet Bill (similar to a Bombshell Bill) at Mario or one of his allies. The bullet explodes on impact, instantly defeating the party member unless they have a instant death prevention item equipped, like a Jinx Belt.
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