Erm, both the Carl Gustav and the RPG-7 has significantly longer ranges than that when shooting against something the size of a merchanter.
But like trav said, that's firing from a pitching fishing boat.
The maximum range of a weapon system is based on the skill of the person wielding that weapon. So I'd imagine the max effective ranges of RPGs, RPKs and AKs would shrink considerably in a typical Somali pirate's hands, especially given the conditions they're firing at.