• Two pieces of different sized plasticard pipe, one that is big enough to slide over the other
• Greenstuff/yellow-blue two part putty
• A (clean!) comb, ideally one with very fine teeth
• Hobby knife
• Poly cement/glue
First, cut your thin pipe to a size you are happy with for the main gun barrel. Cut a short piece of the larger pipe, at an angle on one end. Add glue and slide over the thin pipe, leaving a small tip poking out.
Cut a thin circle from the large pipe, and glue this to the lascannon barrel, somewhere near the middle.
Once fully set, cover the blank end in a layer of greenstuff. Make it no thicker than the large pipe ring from the previous stage. Leave a few millimetres of blank pipe at the end, and smooth the putty with Vaseline and your fingers or a sculpting tool.
Rub a bit of Vaseline on the comb, and start rolling the green stuff-covered pipe over it, apply pressure whilst doing this to create a ribbed cable look.
Allow this to set, clean up the greenstuff, and there you go. A lascannon barrel. At this stage you can add a back section and a power source to the gun, and any details and flourishes of your choosing.