Firing just one of the combi-weapon's weapon?

[Lord Krungharr]

If a unit has combi-weapons, say combi-volkite, can they fire just the volkite part if they want to charge afterwards?  Or do they have to fire the bolter (rapid fire) part too, and thus cannot charge? (unless they're Relentless).

[bushman101]

Yes. The combi rule in the weapon profile says you MAY fire with both, so it's not obligatory.  

The firing rules also go with this. In the Select a Weapon section,  you can select a single weapon mode or choose to not fire with certain models