Figured I'd post this here as well as on my blog.
When Forge World released their new termie combi weapon,s I figured out a way to make a very close replica using a few bits I had lying around.
It really only requires some detailed cutting. The plasticard and greenstuff work (the little bit there is) is optional for those who want to go the distance.
I have the step by step for a flamer, plasma and meltagun conversion.
Here's the whole tutorial on my blog:
http://fromthewarp.blogspot.com/2012/06/how-to-make-terminator-combi-weapons.html