Dan: I use three consecutive sizes of aluminum or brass tubing.
One the largest size I drill the cooling holes in staggered rows using a small V-block and my drill press. I use a 1/8" bit to drill the holes and make sure you use a good sharp bit or the pressure to push the but will dimple the tube and make it difficult to get the barrel which is made out of the next two smaller sizes of tubing into the cooling outer tube that you drilled the holes in.
After you have drilled the outer holes run a drill bit through the center of the outer tube and it will work as a deburring tool.
Remember to drill the holes in a staggered pattern like a checker board. I use a tubing cutter to cut the barrels off and this will slightly roll the outer edge of the two smaller pieces of tube that make up the barrel and it looks very scale because it adds to the thickness of the barrel inside the cooling tube. Cut the barrel tubes at the same time one inside the other. On the cannon which is the center gun I flare the tip with an aul to give it the right shape of the flash surpressor. Each gun only needs to be two inches long so three 12" pieces of tubing will be enough to make up all five guns.. The aluminum is easier to shape but a lot less durable than the brass tubing. I brush the surfaces of the tubes with the little wire brushin my dremmel tool and wipe them down with alcohol before I paint them. If you use the aluminum tubing you don't have to paint the inner barrel. It will polish up easily and look really good. I paint the outer cooling tubes flat black. Hope this helps.

Jack Devine