steel bar to fit the inside dia with rounded ends, slide in as far as you can then roll between two pieces of MDF sheet then push in some more  and repeat.

The grooves in the clamping faces of a Workmate do an excellent job of straightening nearly a metre at a time.

I used Bernard’s method for the 3/8″ dia. tubes for my 5″ga. loco boiler and it worked very well. They were about 8″ long.

John

I used the method I found on utube for coiled 12 mm OD copper tube about 3 m. long. Grip one end in vice jaws the other with Mole grips and wack the Mole grips. It wasn’t perfect but surprisingly effective. You do, of course, loose the flatened tube at each end.

I recall one method, for longer lengths, I think used by a Model Steam Road Vehicle Society member.

He suspended the tube vertically from some suitable point, with a weight on the lower end. Not so heavy it started to draw the tube down, of course, but enough to pull it straight.