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As we can see in this news post, released in February 22nd 2005, the cost was adjusted from 35% -> 55%.
"After months of careful consideration and discussion, we have decided to reduce the effectiveness of the magic shortbow's special attack. Each double shot will now use 55% of the special attack bar, rather than the previous 35%. "
The MSB Special Attack should cost 35% of the bar in the 289 revision, as it is based on January 23rd, 2005.