"I'd contact the manufacturer, Marlin/Glenfield, for a new magazine tube. Can the old tube be straightened out perhaps by driving a rod of the same inner diameter through it?"
I'm going to contact Marlin tomorrow. I thought about pushing something through the mag tube to straighten it but there is a seam on it and believe I would only end up spreading that seam apart. Plus, finding a rod the right diameter will probably prove difficult as well.
