The shotgun was effective because the pirates chose poor tactics and came into shotgun range too soon. It would have been useless beyond 100 yards. A good weapon to have, obviously, once the pirates came inside of its range.
No doubt. Bad guys are rarely good at proper assault tactics. Like you posted earlier, the pirates probably thought their pray was defenseless Euro's.
Not only an effective weapon, but the person using it was obviously proficient using it.
I'm guess they were going to board and terrorize the crew in the hopes of getting any electornic account information / hidden items, then kill the men, and then have fun with the women before killing them.
So they wanted to be able to board a boat with unarmed victims, not a boat with a bunch of bullet holes in it that may be sinking along with all dead bodies.
If you can pretend to be unarmed until they get close to board you (hopefully they want to keep you alive, at least until they figure they have gotten everything from you they can) and then open up - that would be the tactic to use.
That's why you keep 5 rounds of slug in holder on buttstock. Three rounds three inch two rounds three inch magnum.
Hellish big hole and far more damaging than buckshot downrange. Not that accurate at distance but when they hear the sound of that howitzer round hitting their boat you have their attention.