A shield blocking a bullet isn't terribly unrealistic, assuming the shield is metal and fairly thick. The unrealistic part would be reacting fast enough to position the shield between the bullet and yourself. This shouldn't be an issue with an ABS, as I'd assume the player has to be blocking already when the bullet reaches them.
Perhaps, to lend realism to it, you could have bullets deplete some kind of blocking resource (stamina gauge, etc) faster than melee attacks, owing to the sheer speed and unpredictability of them. Maybe that could be a balancing mechanic to discourage turtling against ranged foes. Just a thought.
Of course, if it's pirates, then I don't know how strong their guns would have been. Or whether their shields were metal. Or if they even used shields. Take it with a grain of salt.
