You’re right, I’ve since read the article and they aimed for the heart. Death was pronounced within 3 minutes of the shots. But I don’t think this particular execution used the “random bullet and 2 blanks” method, or at least that’s not a detail mentioned in the article:

“He chose the firing squad knowing that three bullets would shatter his bones and destroy his heart,” said King. “But that was the only choice he had, after the state’s three executions by lethal injection inflicted prolonged and potentially torturous deaths on men he loved like brothers.”