I wouldn't even bring one. They're hideously inaccurate, and you're spending enough points on one that you're cutting into the number of Warriors in the army. Hellcannons look great on paper, but when you actually play them, you're lucky to get a good hit the entire game with one. Bringing two just sort of gives you a reasonable chance at getting a good hit at some point in the game, but it comes at the steep cost of a lot of Warriors, that ultimately will be more useful for the army.
I've found, as a Warriors of Chaos player, it's better to just shrug and accept the lack of shooting in the army, and go with what the army is good at-- being a violent beat-stick. As long as you bring enough Warriors, you're going to be able to get through enemy shooting and still be combat effective, too.