Incitatus, I have restored a few rusty bores in the past. I built the tool shown below that attaches to a 1/2" drill or a 7" grinder. It is adjustable by turning the halves out or in and it uses heavy duty 3" wide cloth sanding belts of appropriate grit for the barrel finish.
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Example: With the 7" grinder attached, I opened up a 2.25" pitted and oval shaped, [out of round by .010"] bore to 2.30" clean and rounded in about 2 1/2 hours. To build this: Use a 3" piece of tubing a 1/4" to 1/2" smaller than the bore, split in half, weld a threaded stud in the center of each tube, connect the halves with an all-thread coupler, weld a [bore length] rod 90 degrees to the center of the coupler. Attach a piece of sand paper with flat head machine screws as shown and wrap the sandpaper around the tube halves away from the direction of rotation. The smaller tubes keep the screw heads away from the bore. Make sure you run it evenly throughout the entire length of the bore for uniformity. You must use heavy duty cloth back sanding belts, regular paper will tear.
Be careful not to adjust too tight as to jam it in the barrel. And with 24 grit paper on the 7" grinder, This thing's a monster and it removes a lot of steel very fast! Dom.
