A plague marine with a Lascannon, egads! In the good old days of Rogue Trader and even to a lesser extent 2nd Edition Nurgle Chaos Space Marines could be armed with heavy weapons. At some point GW decided Death Guard and therefore all Nurgle armies wouldn't use heavy weapons. How this can be a tactical decision made by a Primarch I don't know. The ranges in 40K especially 3rd and 4th Editions are out of whack, there is no way 40,000 years in the future a man can run as far as a pistol can shoot in a brief period of time. Tanks and Heavy Weapons should also shoot much further, but it's only a game...
In the then new Codex rumours strongly suggested that not only will Nurgle Renegades with heavy weapons be legal, they are focussing more on Renegades again and less on the Chaos Legion Chapters. Too bad I stripped about a dozen models armed with heavy weapons and painted as Diseased Sons so as I could repaint them someday as undivided troops.
This is actually the second copy of this model I painted. The first was armed with a Multi-mealta which was made illegal for Chaos before Heavy Weapons were forbidden to Nurgle by the game designers. That model was recently stripped. I actually bought a third version used on eBay and plan to use one those models as the basis for a conversion of a Chaos Lord armed with a Kai Gun for my 3rd tournament army which I'm currently working on.