Get three wedges, that add up to parallel, and beat the centre wedge in to stretch out the tube. There are also sets for expanding muffler tubes. I played around with them at one point in order to come up with something to hold Eccentric BBs. You would need to put something in to protect the steel. You might also need to put a shim in to draw it oversize in one direction.

http://compare.ebay.com/spid/4351987...mTypes&var=srf