On some of the ship missions there is an optional objective of "sink ships by firing on their powder stores".
I know how this works broadly - I've shot up some ships and got the specific target that I can hit with my small gun. The problem is I don't know how you expose the powder stores and much more often I sink the ship before any mini-target appears. For me it seems to be random chance whether you get the powder stores exposed or not but I'm sure that there are ways to do it more precisely.
So is there a technique to exposing a ships powder stores? What is the best way to complete these bonus objectives (so far I've repeated one mission about 10 times because I keep sinking a ship the old fashioned way (or occasionally accidentally ram one and sink it that way).