Heavy Bolters were (and remain) one of my favourite looking 40K weapons. In all incarnations, they look like they could do you a serious nuisance... but the rules are pretty poor. I don't know of anyone who would recommend them for a Space Marine army, and especially not for a Tactical squad.
It occurs that the idea of the Heavy Bolter isn't that it's the GPMG the rules peg it as (not when that job is taken far more effectively by the Assault Cannon), but that it's basically a SAW-style weapon for a Marine Tactical squad. It should provide supressing, rather than eliminating firepower, forcing the enemy to keep their heads down while the Marines advance to butcher them in assault.
So, rather than 36", str 5, ap4, heavy 3, what about 24", str4, ap5, assault 3, pinning? It seems far more tactically useful to me. IG and other Imperial armies with access to Heavy Bolters would keep the old statline. This statline would represent one specially modified for Astartes engagements (much like the IG shotguns is str 3, and the Marine scout shotgun is str 4).
Thoughts?