I've done trial and error for right arm rotation. Rotation goes as follows...
Left side: what rotation looks like in blender.
Right side: what rotation will be in the game.
x+ = y+
-x = -y
y+ = -x
-y = x+
(Z axis breaks hand roation and/or loacation if modified.)
z = z
-z = -z
So, if you wanted to extend the arm to the rotation when a weapon is drawn, rotate negatively on the Y axis.
It's pretty loving confusing and will take a lot of patience. Rather than loading the animation into the game every time, just use shaper for your trial and error.
Edit: okay so I just realized what I said will make no sense so I tried to explain it better. :P