The Combi-Volkite is very fluffy for the XIIth Legion War Hounds as they are a Great Crusade Legion. For them their Veterans would more likely than not to have these in their arsenal.
Now on with the simple (and a bit blurry) build...
I've been asked if removing/ replacing the magazine/ strap from the Combi-Bolter is necessary, and for the bitz to fit together, yes it is, BUT you can just as easily just cut the strap off and use that same magazine.
What do you think? It's pretty straight forward and adds a lot of 'choom' to your army for just a few $