Although I've created a number of combi-plasma guns in the past, I've never exactly been wholly satisfied with the end results. Therefore, I thought I'd try my hand at them again, now that sufficient time has passed to get me over the initial horrors that I created.
One thing that I think makes the difference in my current endeavour is the use of a plasma pistol -- as opposed to a plasma gun. I think the pistol variant has just the correct size for this conversion, and I feel the full gun is simply too long, and certainly looks awkward when placed on top of a regular bolter.
For this conversion, I'm wanting some terminator combi-plasmas. Therefore the other "half" of the gun is a chaos space marine combi-bolter.
I've trimmed off the spiky bits from this bolter, as I wanted to tone it down for use with my Alpha Legion army. The combi-bolter was then literally sawed in half from the barrels downward. As can be seen, there's a gaping air bubble hole in the centre of the plastic, which may surprise some people who are new to the hobby.
For the plasma pistol, I removed the hand and filed down one side of the pistol where I wanted to attach it to the terminator combi-bolter. A little bit of glue later, and a bit of plastic "filling" just where the hand used to be, and its finished and ready for attachment to a new terminator. I will probably go over this with a bit of liquid green stuff to fill in some of the more obvious cracks.
Several more of these and I'll be done!