Well, if you said any of the above, you are dead WRONG.
It is actually:
American 37mm anti-tank pak
This baby contains an ancient metal called Runite that renders the crew more or less impervious to the effects of small-arms fire and high caliber weapons. Bk realism mod displays the awe inspiring ability of the 37mm anti tank pak to resist enemy fire to such a realistic degree that I have to thank Markr for being so thoughtful to include this little known fact about the 37mm in game.
Remember guys, if you want to stay safe, don't hide in a bunker, just hide behind a 37mm anti tank gun, it is more than likely you will survive the skirmish/battle you are in. Thank you for your time.