The best way is to use an aluminum barrel! But most modern kits had done very well with the plastic gun tubes. There is a device called a flex-i-file that assists in making a rounded gun tube. http://www.micromark.com/19-piece-Flex-I-File-Set?gclid=CKG6nYTFptICFVY8gQodOTwC4Q
But, you can also do it carefully with sanding film sold by many modeling companies. This stuff isn't as coarse as regular sand paper. Make sure you sand lightly so you don't make a flat spot.