I've done some antique radio work that required square buss wiring, so have a coil of it. Naturally, it should be arrow straight with nice tight corners. To uncoil it, I used several methods, one of which was pretty effective and already mentioned. I clamped one end in a mill vise and instead of pliers, I wrapped it around a heavy bar and gave it a couple of stout jerks. It straightened it nicely. But for more control, I used another method. I again clamped one end in a mill vise, but the other end I clamped in a toolholder on a lathe that was reasonably close to being in line, and used that to stretch the wire. It can out remarkably straight. I then kept it taped to a 1 x 4 until I needed it. I still have some, and it is still straight, along with what is left of the coil.