This might be old news, but I just found it. This guy worked on Babylon 5 and now the BSG series. He has some really cool things on his site about the old school Cylons from Razor:
http://darthmojo.wordpress.com/2008/05/12/bsg-vfx-anatomy-of-a-cylon/
Great shot of the Cylon gun on this page. It looks like it's based on a M60 machine gun:
http://darthmojo.wordpress.com/2008/05/13/bsg-vfx-anatomy-of-a-cylon-extras/
http://darthmojo.wordpress.com/2008/05/12/bsg-vfx-anatomy-of-a-cylon/
Great shot of the Cylon gun on this page. It looks like it's based on a M60 machine gun:
http://darthmojo.wordpress.com/2008/05/13/bsg-vfx-anatomy-of-a-cylon-extras/