Use Heavy Bolters!!!
Autocannons are nice but you are BS3 so having 9 shots means more 3 more potential hits per turn than the Autocannons can pump out. Yes S7 is useful against light tanks but if you are relying on your heavy weapons teams to hunt tanks then you're reading the IG all wrong.
Heavy Bolters are very adaptable. They'll mow down scads of infantry and still put a dent into a big critter by sheer weight of fire. Even against enemy transports the sheer number of shots will almost certainly earn you a glancing hit.
Also enemies often overlook Heavy Bolter teams, aiming for the more expensive and supposedly better stuff in the early part of the game. By the middle they are wondering why their transports are burning and their assault teams are eating dirt. By the end, if they survive, the Heavy Bolters are hosing enemy troops off objectives.
I generally have one Heavy Bolter support squad, and also place one in each Infantry Squad as well. This means that if I can draw the enemy into my kill sack I can drop 18 S5 shots on their traitorous/xenos *****. Makes me come over all Tau
In fairness I also have an Autocannon support squad as well because they are good against transports and the lighter tanks (and they draw fire away from my HB's).