Shunt wire might refer to the current sensing resistor.
Although I dont know specifics of this particular unit a typical meter movement is 1ma fs (Full Scale).
The shunt converts this fs to 50A (or whatever) by "shunting" most of the current past the meter.
The value of the shunt is determined by the ratio of coil resistance of meter to shunt and meter fs sensitivity to the desired fs.
The ideal ammeter would have 0 ohms resistance, the shunt for a 50A meter will be lower than many ohmmeters can resolve so I am not sure what "the ammeter shorted out" means.
Now if the "meter" was on the pcb and the shunt was external and -opened- then the high current might have melted traces on the board depending on circuit design.
BTW: The above is for an analog meter, in a digital meter the (very low) voltage drop accross the shunt is used to measure current.