The easy way to make a coilgun barrel is to go back to basics. Before the advent of plastic cores in transformers and inductors, the cores were made by wrapping lacquered paper around a dowel or shaped mould the desired size. A simple coating of wax on the dowel or mould will keep the core from sticking as it dries, and if it does stick you can loosen it with a little heat from a hair dryer. For strength wrap the first layer in one direction and let dry then wrap the second layer in the opposite direction repeat this until the barrel is the desired thickness. If you do not let the layers dry between applying layers it will take forever for the barrel to dry unless you use a resin based lacquer. This makes a strong lightweight barrel the size you desire that does not promote eddy currents.