Seeing bullets drop down due to gravity or shatter when shot into water(it is impossible to kill someone who is under water with assault rifle). Pistol bullets move with slower speed and thus can kill/hurt someone about 1m under water, same with some other weapons that have slower moving bullets.
Also i would love to see bullets penetrating different materials. Like assault rifle going through large tree trunk while not being able to penetrate large jungle tree trunk types, which can of course could be penetrated by larger caliber weapon/assault rifle.
This also could give some diversity to weapons, like having scar it would fire faster and would be more accurate than FY71 but would lack firepower when trying to penetrate concrete wall while larger caliber FY71 would kill enemy behind concrete wall easily.