Get a good, all metal choke tube wrench. Hold the barrel so the choke is pointing up, and put a firm tension on it in the direction of loosening. Have smeoen take a small hammer, and tap rapidly on the end of the wrench. Reverse direction and tap again. if you can even get it to move an 1/8th inch, you are in business. Keep tightening/loosening while your buddy keeps tapping with the hammer. you will gain more and more each time. After you can get it to turn one revolution, re apply penetrating oil. Keep tapping each time. Even when you can force the wrench without tapping, don't do it or you can bugger the treads. The tapping and back and forth motion keeps breaking down the rust and saves the threads. I know it is slow, but if you save the barrel, it is worth it.
FWIW.........Kroil puts PB to shame.