All my what.
First of all I would just house rule the shooting in combat. The crewmen are far too busy fighting the attackers and don't have time to load the gun.
I think you've got the volley of shots thing wrong. Volley of shots is the name of the helblasters shooting attack. The War Machine Crew Table gives you the TO HIT roll depending on how many crew members are alive (or within 1" of the war machine.). So if you have all 3 members near the gun, you hit on 3+. If you only have 1 crew member, you hit on 5+.
Every time you fire you must choose how many decks you fire, the number of crewmen has no effect on this. If you roll doubles, the gun jams and doesn't fire this turn. If you have a model with the Human Engineer keyword within 1" of the gun you can reroll all the dice determining the amount of shots. Also should reduce jamming that way.