"Plunder a hunter ship" (Buy Broadside Cannons) is an optional mission objective in Sequence 3 / Memory 04: Raise the Black Flag
Walkthrough
As you plunder more merchant ships, a your Wanted Level will rise and eventually Hunter Ships will start appearing and chasing you down
Plunder one of those to complete this optional objective
Be careful though, Hunter Ships possess superior weapons and a stronger hull than an average ship