You can also use 1/35th scale assault rifles, the overall size is very similar to the GW weapons, but they aren't as bulky, which certainly says something for the size of GW guns.... I have a pack somewhere that had AK74's in it, I gave them straight clips instead of the curved one and they fit in fine. Also, 50cal's etc make great heavy stubbers. Or MG42's etc if you're after the better made ones such as Van Saar or Escher.
Alternatively, The Assault Group has a very good range of modern weapons that are in the right size and bulk, many of which would make great auto-guns.