So I have an Assault on Black Reach dreadnought, but I've always used the multi-melta arm as a plasma cannon stand in. Since I got the dark vengeance set, I realized that I have one more plasma cannon than I need, so I figured it was time to make a real plasma cannon. There is a great tutorial that gave me the idea to do this over on
Da Rollin Waaagh!. I'll give a quick rundown of what I did, but if you want a more in depth tutorial, head over to his post. More below...