How to straighten copper tubing?

I'm making an air "cannon", but my tubing is bent. It is 3/8 copper tubing, and is very bendy. Is there a way to get it straight, or am I better off buying a small steel tube? (I did try googling this, with no helpful results)

Place your tube on a hard and flat surface. Place 1ft length of a hard wood board (about 2x4 size) across the tube and use it to roll it back and forth while pressing down. It may help to rotate the board between 45° and 90° to the pipe as you roll it. Depending on how kinked up your tube is, you might need to slap it lightly with the board to get it straight enough to roll reliably.