First off, my apologies if this idea has already been discussed to death.
So, as we all know, the bolter is one of the most controversial aspects of DW. Some like the idea that SM weapons are more powerful than the versions used by the rest of the Imperium, others dislike it. It also raises balance issues.
As I recall, one of the reasons given for the high damage output was the need to overcome the intrinsic damage-soaking ability of other SMs. As such, bolters can currently hurt SM, while (in the eyes of many) providing total overkill for most other targets, and rendering several of the other weapon choices largely irrelevant.
So, here's a possibility: why not give bolters the Felling attribute? That way, you could lower the damage stats w/o rendering SM completely invulnerable to it.
While this could be seen as removing the applicability of Vengeance rounds, I would suggest that Vr could simply raise Pen to 9 and retain their risk of overheating.
Of course, this provides somewhat of a domino effect, as this change would in turn infringe on the purpose of Kraken rounds. I would suggest dropping the Pen of Kraken rounds to 6; this would preserve their purpose of "longer range with a bit more punch," as well as giving them an effective 'AP' of 4 (ignoring carapace armor), which better mirrors their tabletop counterparts (for anyone who takes such things into consideration).
So, what are your thoughts? What balance concerns would this address, and what issues would it exacerbate?