What about tweaking the Autofire rules a bit? As far as I see it, you've got two lines of thought:
1. Cinematic: levelling an autofire weapon and stiching a target with hot lead.
2. Realism: an automatic weapon is used primarily to create an AoE kill-zone.
I am of the mind that Autofire doesn't make you more accurate, but I can see how they designed the rule with that hefty bonus to include abstract cinematics and realism all in one easy rule.
So, here's my suggestions:
1. Reduce the bonus to +10 BS
2. Unless I'm mistaken, bonuses to hit stack in DW. Maybe 'Highest bonus applies' would work better for balance. In this case, reducing the Autofire bonus to hit would not be necessary. I've had situations where the Dev is getting a BS of well over 100, so no wonder he gets 7 hits on a Tyrant and demolishes the poor bastard in a round or two of shooting.
3. Heavy Bolters fire heavier versions of the normal bolt shell. Maybe the HB stats need a tiny amount of tweaking. Looking in the book, though, I can't see much to tweak, so maybe the Autofire and bonus system needs some tinkering instead.
Also might be worth considering that HB's are not essentially machine guns. Not to get too anal about it, but in a sense, Bolt weapons pretty much fire straight-line grenades, hence the low RoF. Doesn't make much sense compared to what we have nowadays, but I guess you have to oversee those small artistic licenses, for the flavour of the game.
. I'd give it something like on a 1 it decreases by 2m, 2-5 decreases by 1m, 6-9 increases by 1m, and 10 increases by 2m. Or give it a tiered system, where it's more likely to increase on rounds 1-3, then more likely to decrease after that.