A spool gun has to be run on a constant voltage machine, so a MIG machine.
I have a 15A spoolgun I used on a Millermatic 251. This set up is nice because you can adjust your inrush wirefeed speed. With a MIG welder, the power supply is designed to supply as much current as needed to maintain the voltage you have set. When starting the arc, the moment the wire touches the work, the current goes from zero to maximum deliverable by the power supply. The current on a MIG is NOT adjustable by itself, the wire feed speed actually helps determine the current a MIG supply will put out. With aluminum, particularly if your stickout distance from the tip is a little to long, the maximum current travels through the wire and 'vaporizes' it before it gets a chance to get an arc started on your work piece. This also burns the wire back into the tip, so you either replace the tip or get the aluminum dug out of it somehow. By adjusting the wire feed arc initiation speed, the wire is fed slower until the arc starts, and once the arc is going, the wire speed goes to the setting dialed in.
