Searing Barrage

Searing Barrage's adamant ability causes it to deal additional damage. It doesn't replace the damage dealt to the target creature.

If the target creature is an illegal target by the time Searing Barrage tries to resolve, the spell won't resolve. No player will be dealt damage, even if you spent three red mana.

Adamant effects check what mana was actually spent to cast a spell. If an effect allows you to spend mana “as though it were mana” of any color or type, that allows you to spend mana you couldn't otherwise spend, but it doesn't change what mana you spent to cast the spell.