User blog comment:NickyLinnea/Fortune Hunter upgrade on a War Frigate/@comment-3115143-20111109191923/@comment-83.108.133.100-20111109221817

The 15% is the chance each broadside shot has to be the "special" ammo of that hull upgrade. That means on average, for every 100 broadside shots, 15 of them will be a special ammo. The shot does the same damage as it would do if it was fired from a deck cannon. Don't know yet if it's affected by your cannon skills.