Reasons for which you might
not want this include the fact the game has a pretty extensive set of rules, and as far as I've seen people 'round here tend not to bother with that type of thing. Also I've given certain reasons to question my reliability as a GM in the past, however now I have a whole campaign written up in detail - two campaigns, even!
What could go wrong Okay, no, I'm not saying this, but it's implied.
I have the core rulebook in my dropbox
here. I also have a few supplements if anybody needs those -
here. Only War has a few fundamental differences from, say, Dark Heresy - there are no Career Ranks, instead you get Aptitudes for shiznit wot make it easier to learn, determined by your Specialty, which is like a class but not. Plus each PC gets Comrades, which is the GW-approved term for "redshirts". You get to make those guys too, and they help you with dragging the autocannon around, that sorta thing. There is also a Logistics system for requisitioning gear, meaning that the Munitorum can issue you a crate of Underwear, Female, Ogryn-Sized, instead of your breaching charges or jump packs.
This 1d4chan article sheds some light on the whole matter.
The Lexicanum provides pertinent info on various Warhammer 40000 - related topics. There are also some rather good book series about the Imperial Guard, Abnett's
Gaunt's Ghosts and Mitchell's
Ciaphas Cain, Hero of the Imperium chief among them. You can give those a try to get a feel for the athmosphere.
Another important thing is that you get to choose the regiment your PCs belong to, or create your own. If enough (about five, I'd say) people express interest in playing this, then I'll compile a list of available regiments, or the aforementioned interested persons can create their own regiment through a vaguely democratic process, as the rulebook suggests. Then I'll take a week or two to give the campaign a final polish, and start a thread for the game itself.
So, any volunteers?