Camera and weapon cant in lean; this is mentioned briefly in forum threads here:

http://forums.bistudio.com/showthread.php?132395-Cover-Systems-Lean-amp-Realism

And in more detail here:

http://forums.bistudio.com/showthread.php?155004-Correct-Weapon-Handling-amp-Canting

This is technically, metrically wrong in terms of proper weapon handling and perspective -- the world does not 'tilt' if we cant our body, head, or a rifle separately or in unison.

Canting an aimed firearm from any fire position is improper weapon handling, and is rigorously repudiated and corrected in virtually all firearms training military and civilian.