Sorry if this was mentioned before but i dont think lowering the gun is anything you have control over. If you look at the video with the Priest guy in it, Gordon lowers his gun whenever the crosshairs are on him... probably a way to prevent you from killing plot vital characters
edit: I...
I always start on medium... I dont feel immersed if its too easy. There has to be some challenge and fear =) If the SP is really good ill play though on hard too.
I think the ones that got mowed down by the PDW died alot cooler than the ones blasted by the shotgun. It looked like they "poofed" in a cloud of green smoke or something with the main part of their body disappearing when shot head-on by the SG