Hey dude, check it out: my most recent MOC has a version of your chain gun in it! Please look at it, there's a couple pics singling the particular clone out from the rest of the group, so you can get a better view of it. It's supposed to be like the kind in Star Wars: Battlefront II, where you can mount it on your shoulder :)
I will add some instructions for the chain gun if you so like... I not did have rex or another clone trooper so I could not make 5s, yeah this is one of my favs!!! I know it does suck just a little that heavy dies... but he went down by taking battalion of droids with him.