You need the cannon pieces to create one.
No, you can not.
the dwarf multicannon stays where it stands
to use the dwarf multicannon, complete quest dwarf cannon. to use the hand cannon, complete the giant dwarf. those two quests befriend dwarfs.
You can get a dwarf multicannon after the completion of the 'Dwarf Cannon' quest, once this is completed this quest you can purchase a cannon from the dwarf engineer on ice mountain for 750,000gp.
There is no such potion. And it isn't relevant: the Dwarven Multicannon can't be used to combat against other players, only against NPCs.
Nulodion is the official dwarf multicannon expert. During the Dwarf Cannon quest you meet Nulodion who helps you during the quest.You can locate him near the Entrance to Dwarven Mine on Ice Mountain, north of Falador.
in the cave next to the fishing guid.
The Dwarf multicannon degrades after around 2000 shots. When this happens, you can just take it to Nulodion near the Dwarven Mine and he will repair it for free. Your cannon will never go away unless you die with it.
north of falador in front of dwarven mine
You cannot hit any higher with it. The max hit is 300 I think you may hit more often depending on your range level though. The multicannon doesn't show any stats nor does any range equipment effect the hit rate or hit power of the cannon. However a good technique with using the cannon is ranging alongside with the cannon so you definitely gain faster experience in range.
And what is the question?
he is in the cave by fishing guild. you search boxes and crates down there until you find him. he is usually in the room with the beds but he could be anywhere down there.