I assume you're already using music wire? That's the usual option for springs.
For a lot of springs you can get away with no further heat treating to the music wire but for extreme duty applications they need heat further heat treatment to "set" the new form following winding.
Another option is to cold form the music wire to help work harden it a bit before it goes onto the mandrel you're using. For years I made my own springs for small things here and there. I used to just hold the wire in a leather glove and pull back on it while winding. But recently from looking around on some metal working forums I made up a gizmo that tensions the music wire by passing it around a set of three pins that tensions the wire by passing it through a zigzag path of three pins that have highly polished U grooves turned int them. This seems to help work the metal so that the springs I've made with this tool are more resilient. I'm actually using hammer springs made with this gizmo on two of my guns so far and they are holding up well. Not sure on the very long life issue but with the cost being so low I don't mind making new ones here and there.
I know that for some wires the factories put them through a passivating heat treatment process. This is different from a full on heat to red, quench then temper sort of deal. It's also limited to springs made from special materials. For most regular music wire springs they are just used as they come off the winder with no further treatment that I could learn about.
You can see the gizmo in the picture I attached to this thread....
http://www.canadiangunnutz.com/forum/showthread.php?p=3531206#post3531206
Of course you'll need a MUCH bigger hole for your magazine mandrel so some playing around will be needed.
The real challenge is to be able to form a spring that has enough coils or from a light enough wire that it will collapse to the one inch while not overstressing the wire coils enough to set to a shorter free length. That normally comes from using more turns or a finer guage. But more turns or a finer gauge reduces the spring rate and you may not have enough force to work the cartridges to move correctly. Going to a heavier gauge to get a stronger spring may not let you have the right free and coil bound lengths.
One option that you may want to play with is to make the spring a conical format rather than cylindrical. With a conical format the spring can collapse to a flat plate format instead of a short cylinder. Or even if the coil spacing isn't sufficient to collapse to a flat plate you can still use more turns that results in a shorter conical coil bound form that sits shorter than with a regular straight cylinder form.
