This mod converts BF1942 to Warhammer 40K. It is a total conversion, but is based off of the pen and paper game (the original Warhammer 40K), not the video game. I expect it will be more authentic. I haven't played it though.
Battlefield 40k is a total conversion modification of Electronic Art's/DICE's 3D FPS Infantry & Vehicular Fighting Action game "Battlefield 1942, 2 and 2142 "
based on Games Workshop's table top wargame "Warhammer 40,000"