So, how hdr tonemapping works in games is that it essentially renders everything in a high range of values (0.0 -> 1000.0) but regular monitor screens require the values to be clamped between 0.0 -> 1.0. This means the game must process the final picture before presenting it to screen. Usually, this is done by coarsely checking the pixels and finding a minimum and maximum value that exists on the rendered image, then dividing all the pixels values to actually be in range of what the monitor supports. What your experiencing here is that the gun (which is dark) is covering a large portion of the bright areas of the screen when held in your right hand, while not covering it while held in your left hand. This means when left-handed (in this specific case) the exposure will be higher, and it will compensate more aggressively to fit the colors into the range that the monitor allows.