http://www.military-sf.com/index.htm
This website may have been posted before, but I was recently made aware of this index created by a former U.S Marine who served in Desert Storm and Desert Shield. It is a great way to look at how to convincingly write combat situations, which comes in handy with any Warhammer
40K or similar Science Fiction setting. It even has a recommended readings list to see how other authors have applied military knowledge to their writing.
I found the sections on Mortars and Firefight Dynamics very interesting. It sounds great for writing Guard versus renegade situations, and knowing the rules of engagement allows us to break them when we throw an alien like an Ork or Eldar into the fray.